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usb/gadget: warning in ep_write_iter/__alloc_pages_nodemask

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  usb/gadget: warning in ep_write_iter/__alloc_pages_nodemask Andrey Konovalov <andreyknvl@google.com> - 2016-12-12 21:40 +0100
    Re: usb/gadget: warning in ep_write_iter/__alloc_pages_nodemask Andrey Konovalov <andreyknvl@google.com> - 2016-12-12 21:40 +0100
      Re: usb/gadget: warning in ep_write_iter/__alloc_pages_nodemask Alan Stern <stern@rowland.harvard.edu> - 2016-12-12 22:10 +0100
      Re: usb/gadget: warning in ep_write_iter/__alloc_pages_nodemask Michal Hocko <mhocko@kernel.org> - 2016-12-12 22:10 +0100
        Re: usb/gadget: warning in ep_write_iter/__alloc_pages_nodemask Alan Stern <stern@rowland.harvard.edu> - 2016-12-12 22:20 +0100
          Re: usb/gadget: warning in ep_write_iter/__alloc_pages_nodemask Michal Hocko <mhocko@kernel.org> - 2016-12-13 09:10 +0100
            Re: usb/gadget: warning in ep_write_iter/__alloc_pages_nodemask Alan Stern <stern@rowland.harvard.edu> - 2016-12-13 14:40 +0100
              Re: usb/gadget: warning in ep_write_iter/__alloc_pages_nodemask Michal Hocko <mhocko@kernel.org> - 2016-12-14 10:20 +0100
                Re: usb/gadget: warning in ep_write_iter/__alloc_pages_nodemask Michal Hocko <mhocko@kernel.org> - 2016-12-14 17:20 +0100
                Re: usb/gadget: warning in ep_write_iter/__alloc_pages_nodemask Alan Stern <stern@rowland.harvard.edu> - 2016-12-14 17:50 +0100

#1540625 — usb/gadget: warning in ep_write_iter/__alloc_pages_nodemask

Hi!

While running the syzkaller fuzzer I've got the following error report.

The issue is that the len argument is not checked for being too big.

WARNING: CPU: 1 PID: 9935 at mm/page_alloc.c:3511
__alloc_pages_nodemask+0x159c/0x1e20
Kernel panic - not syncing: panic_on_warn set ...

CPU: 1 PID: 9935 Comm: syz-executor0 Not tainted 4.9.0-rc7+ #34
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
 ffff88006949f2c8 ffffffff81f96b8a ffffffff00000200 1ffff1000d293dec
 ffffed000d293de4 0000000000000a06 0000000041b58ab3 ffffffff8598b510
 ffffffff81f968f8 0000000041b58ab3 ffffffff85942a58 ffffffff81432860
Call Trace:
 [<     inline     >] __dump_stack lib/dump_stack.c:15
 [<ffffffff81f96b8a>] dump_stack+0x292/0x398 lib/dump_stack.c:51
 [<ffffffff8168c88e>] panic+0x1cb/0x3a9 kernel/panic.c:179
 [<ffffffff812b80b4>] __warn+0x1c4/0x1e0 kernel/panic.c:542
 [<ffffffff812b831c>] warn_slowpath_null+0x2c/0x40 kernel/panic.c:585
 [<     inline     >] __alloc_pages_slowpath mm/page_alloc.c:3511
 [<ffffffff816c08ac>] __alloc_pages_nodemask+0x159c/0x1e20 mm/page_alloc.c:3781
 [<ffffffff817cde17>] alloc_pages_current+0x1c7/0x6b0 mm/mempolicy.c:2072
 [<     inline     >] alloc_pages include/linux/gfp.h:469
 [<ffffffff8172fd8f>] kmalloc_order+0x1f/0x70 mm/slab_common.c:1015
 [<ffffffff8172fdff>] kmalloc_order_trace+0x1f/0x160 mm/slab_common.c:1026
 [<     inline     >] kmalloc_large include/linux/slab.h:422
 [<ffffffff817e01f0>] __kmalloc+0x210/0x2d0 mm/slub.c:3723
 [<     inline     >] kmalloc include/linux/slab.h:495
 [<ffffffff832262a7>] ep_write_iter+0x167/0xb50
drivers/usb/gadget/legacy/inode.c:664
 [<     inline     >] new_sync_write fs/read_write.c:499
 [<ffffffff817fdcd3>] __vfs_write+0x483/0x760 fs/read_write.c:512
 [<ffffffff817ff720>] vfs_write+0x170/0x4e0 fs/read_write.c:560
 [<     inline     >] SYSC_write fs/read_write.c:607
 [<ffffffff81803b2b>] SyS_write+0xfb/0x230 fs/read_write.c:599
 [<ffffffff84f47ec1>] entry_SYSCALL_64_fastpath+0x1f/0xc2
Dumping ftrace buffer:
   (ftrace buffer empty)
Kernel Offset: disabled

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#1540627

On Mon, Dec 12, 2016 at 9:31 PM, Andrey Konovalov <andreyknvl@google.com> wrote:
> Hi!
>
> While running the syzkaller fuzzer I've got the following error report.
>
> The issue is that the len argument is not checked for being too big.
>
> WARNING: CPU: 1 PID: 9935 at mm/page_alloc.c:3511
> __alloc_pages_nodemask+0x159c/0x1e20
> Kernel panic - not syncing: panic_on_warn set ...
>
> CPU: 1 PID: 9935 Comm: syz-executor0 Not tainted 4.9.0-rc7+ #34
> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
>  ffff88006949f2c8 ffffffff81f96b8a ffffffff00000200 1ffff1000d293dec
>  ffffed000d293de4 0000000000000a06 0000000041b58ab3 ffffffff8598b510
>  ffffffff81f968f8 0000000041b58ab3 ffffffff85942a58 ffffffff81432860
> Call Trace:
>  [<     inline     >] __dump_stack lib/dump_stack.c:15
>  [<ffffffff81f96b8a>] dump_stack+0x292/0x398 lib/dump_stack.c:51
>  [<ffffffff8168c88e>] panic+0x1cb/0x3a9 kernel/panic.c:179
>  [<ffffffff812b80b4>] __warn+0x1c4/0x1e0 kernel/panic.c:542
>  [<ffffffff812b831c>] warn_slowpath_null+0x2c/0x40 kernel/panic.c:585
>  [<     inline     >] __alloc_pages_slowpath mm/page_alloc.c:3511
>  [<ffffffff816c08ac>] __alloc_pages_nodemask+0x159c/0x1e20 mm/page_alloc.c:3781
>  [<ffffffff817cde17>] alloc_pages_current+0x1c7/0x6b0 mm/mempolicy.c:2072
>  [<     inline     >] alloc_pages include/linux/gfp.h:469
>  [<ffffffff8172fd8f>] kmalloc_order+0x1f/0x70 mm/slab_common.c:1015
>  [<ffffffff8172fdff>] kmalloc_order_trace+0x1f/0x160 mm/slab_common.c:1026
>  [<     inline     >] kmalloc_large include/linux/slab.h:422
>  [<ffffffff817e01f0>] __kmalloc+0x210/0x2d0 mm/slub.c:3723
>  [<     inline     >] kmalloc include/linux/slab.h:495
>  [<ffffffff832262a7>] ep_write_iter+0x167/0xb50
> drivers/usb/gadget/legacy/inode.c:664
>  [<     inline     >] new_sync_write fs/read_write.c:499
>  [<ffffffff817fdcd3>] __vfs_write+0x483/0x760 fs/read_write.c:512
>  [<ffffffff817ff720>] vfs_write+0x170/0x4e0 fs/read_write.c:560
>  [<     inline     >] SYSC_write fs/read_write.c:607
>  [<ffffffff81803b2b>] SyS_write+0xfb/0x230 fs/read_write.c:599
>  [<ffffffff84f47ec1>] entry_SYSCALL_64_fastpath+0x1f/0xc2
> Dumping ftrace buffer:
>    (ftrace buffer empty)
> Kernel Offset: disabled

+syzkaller

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#1540652

On Mon, 12 Dec 2016, Andrey Konovalov wrote:

> On Mon, Dec 12, 2016 at 9:31 PM, Andrey Konovalov <andreyknvl@google.com> wrote:
> > Hi!
> >
> > While running the syzkaller fuzzer I've got the following error report.
> >
> > The issue is that the len argument is not checked for being too big.
> >
> > WARNING: CPU: 1 PID: 9935 at mm/page_alloc.c:3511
> > __alloc_pages_nodemask+0x159c/0x1e20
> > Kernel panic - not syncing: panic_on_warn set ...
> >
> > CPU: 1 PID: 9935 Comm: syz-executor0 Not tainted 4.9.0-rc7+ #34
> > Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
> >  ffff88006949f2c8 ffffffff81f96b8a ffffffff00000200 1ffff1000d293dec
> >  ffffed000d293de4 0000000000000a06 0000000041b58ab3 ffffffff8598b510
> >  ffffffff81f968f8 0000000041b58ab3 ffffffff85942a58 ffffffff81432860
> > Call Trace:
> >  [<     inline     >] __dump_stack lib/dump_stack.c:15
> >  [<ffffffff81f96b8a>] dump_stack+0x292/0x398 lib/dump_stack.c:51
> >  [<ffffffff8168c88e>] panic+0x1cb/0x3a9 kernel/panic.c:179
> >  [<ffffffff812b80b4>] __warn+0x1c4/0x1e0 kernel/panic.c:542
> >  [<ffffffff812b831c>] warn_slowpath_null+0x2c/0x40 kernel/panic.c:585
> >  [<     inline     >] __alloc_pages_slowpath mm/page_alloc.c:3511
> >  [<ffffffff816c08ac>] __alloc_pages_nodemask+0x159c/0x1e20 mm/page_alloc.c:3781
> >  [<ffffffff817cde17>] alloc_pages_current+0x1c7/0x6b0 mm/mempolicy.c:2072
> >  [<     inline     >] alloc_pages include/linux/gfp.h:469
> >  [<ffffffff8172fd8f>] kmalloc_order+0x1f/0x70 mm/slab_common.c:1015
> >  [<ffffffff8172fdff>] kmalloc_order_trace+0x1f/0x160 mm/slab_common.c:1026
> >  [<     inline     >] kmalloc_large include/linux/slab.h:422
> >  [<ffffffff817e01f0>] __kmalloc+0x210/0x2d0 mm/slub.c:3723
> >  [<     inline     >] kmalloc include/linux/slab.h:495
> >  [<ffffffff832262a7>] ep_write_iter+0x167/0xb50
> > drivers/usb/gadget/legacy/inode.c:664
> >  [<     inline     >] new_sync_write fs/read_write.c:499
> >  [<ffffffff817fdcd3>] __vfs_write+0x483/0x760 fs/read_write.c:512
> >  [<ffffffff817ff720>] vfs_write+0x170/0x4e0 fs/read_write.c:560
> >  [<     inline     >] SYSC_write fs/read_write.c:607
> >  [<ffffffff81803b2b>] SyS_write+0xfb/0x230 fs/read_write.c:599
> >  [<ffffffff84f47ec1>] entry_SYSCALL_64_fastpath+0x1f/0xc2
> > Dumping ftrace buffer:
> >    (ftrace buffer empty)
> > Kernel Offset: disabled

I'm not an expert in this area, but it seems like length checking of
I/O operations should be done in a more central location, like the
VFS, rather than in a million different drivers.

Anyway, it's not a big deal if the memory allocation fails.  Users who
try to transfer large amounts of data at once should expect that
sometimes it won't work.

Alan Stern

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#1540661

On Mon 12-12-16 21:32:35, Andrey Konovalov wrote:
> On Mon, Dec 12, 2016 at 9:31 PM, Andrey Konovalov <andreyknvl@google.com> wrote:
> > Hi!
> >
> > While running the syzkaller fuzzer I've got the following error report.
> >
> > The issue is that the len argument is not checked for being too big.

Well, the value is checked in kmalloc_slab. There is a discrepancy
though. While the page allocator enforces order < MAX_ORDER, slab
allocators enforce size <= KMALLOC_MAX_SIZE and

KMALLOC_MAX_SIZE is 1UL << (MAX_ORDER + PAGE_SHIFT) for SLUB which is
what I expect you have or 1UL << (MAX_ORDER + PAGE_SHIFT - 1) for SLAB
on most archs.  This means that KMALLOC_MAX_SIZE might be MAX_ORDER for
SLUB which would explain your warning.

Let's CC Christoph, this is nothing really new, but I suspect that SLUB
should change the maximum size to something like SLAB does.

That being said, what ep_write_iter does sounds quite stupit. It just
allocates a large continuous buffer which seems to be under user
control...  Aka no good! It should do that per pages or something like
that. Something worth fixing

> > WARNING: CPU: 1 PID: 9935 at mm/page_alloc.c:3511
> > __alloc_pages_nodemask+0x159c/0x1e20
> > Kernel panic - not syncing: panic_on_warn set ...
> >
> > CPU: 1 PID: 9935 Comm: syz-executor0 Not tainted 4.9.0-rc7+ #34
> > Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
> >  ffff88006949f2c8 ffffffff81f96b8a ffffffff00000200 1ffff1000d293dec
> >  ffffed000d293de4 0000000000000a06 0000000041b58ab3 ffffffff8598b510
> >  ffffffff81f968f8 0000000041b58ab3 ffffffff85942a58 ffffffff81432860
> > Call Trace:
> >  [<     inline     >] __dump_stack lib/dump_stack.c:15
> >  [<ffffffff81f96b8a>] dump_stack+0x292/0x398 lib/dump_stack.c:51
> >  [<ffffffff8168c88e>] panic+0x1cb/0x3a9 kernel/panic.c:179
> >  [<ffffffff812b80b4>] __warn+0x1c4/0x1e0 kernel/panic.c:542
> >  [<ffffffff812b831c>] warn_slowpath_null+0x2c/0x40 kernel/panic.c:585
> >  [<     inline     >] __alloc_pages_slowpath mm/page_alloc.c:3511
> >  [<ffffffff816c08ac>] __alloc_pages_nodemask+0x159c/0x1e20 mm/page_alloc.c:3781
> >  [<ffffffff817cde17>] alloc_pages_current+0x1c7/0x6b0 mm/mempolicy.c:2072
> >  [<     inline     >] alloc_pages include/linux/gfp.h:469
> >  [<ffffffff8172fd8f>] kmalloc_order+0x1f/0x70 mm/slab_common.c:1015
> >  [<ffffffff8172fdff>] kmalloc_order_trace+0x1f/0x160 mm/slab_common.c:1026
> >  [<     inline     >] kmalloc_large include/linux/slab.h:422
> >  [<ffffffff817e01f0>] __kmalloc+0x210/0x2d0 mm/slub.c:3723
> >  [<     inline     >] kmalloc include/linux/slab.h:495
> >  [<ffffffff832262a7>] ep_write_iter+0x167/0xb50
> > drivers/usb/gadget/legacy/inode.c:664
> >  [<     inline     >] new_sync_write fs/read_write.c:499
> >  [<ffffffff817fdcd3>] __vfs_write+0x483/0x760 fs/read_write.c:512
> >  [<ffffffff817ff720>] vfs_write+0x170/0x4e0 fs/read_write.c:560
> >  [<     inline     >] SYSC_write fs/read_write.c:607
> >  [<ffffffff81803b2b>] SyS_write+0xfb/0x230 fs/read_write.c:599
> >  [<ffffffff84f47ec1>] entry_SYSCALL_64_fastpath+0x1f/0xc2
> > Dumping ftrace buffer:
> >    (ftrace buffer empty)
> > Kernel Offset: disabled
> 
> +syzkaller

-- 
Michal Hocko
SUSE Labs

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#1540670

On Mon, 12 Dec 2016, Michal Hocko wrote:

> On Mon 12-12-16 21:32:35, Andrey Konovalov wrote:
> > On Mon, Dec 12, 2016 at 9:31 PM, Andrey Konovalov <andreyknvl@google.com> wrote:
> > > Hi!
> > >
> > > While running the syzkaller fuzzer I've got the following error report.
> > >
> > > The issue is that the len argument is not checked for being too big.
> 
> Well, the value is checked in kmalloc_slab. There is a discrepancy
> though. While the page allocator enforces order < MAX_ORDER, slab
> allocators enforce size <= KMALLOC_MAX_SIZE and
> 
> KMALLOC_MAX_SIZE is 1UL << (MAX_ORDER + PAGE_SHIFT) for SLUB which is
> what I expect you have or 1UL << (MAX_ORDER + PAGE_SHIFT - 1) for SLAB
> on most archs.  This means that KMALLOC_MAX_SIZE might be MAX_ORDER for
> SLUB which would explain your warning.
> 
> Let's CC Christoph, this is nothing really new, but I suspect that SLUB
> should change the maximum size to something like SLAB does.
> 
> That being said, what ep_write_iter does sounds quite stupit. It just
> allocates a large continuous buffer which seems to be under user
> control...  Aka no good! It should do that per pages or something like
> that. Something worth fixing

It's not important enough to make the driver do all this work.  If
users want to send large amounts of data, they can send it a page at a
time (or something like that).

If you really want to prevent the driver from attempting to allocate a
large buffer, all that's needed is an upper limit on the total size.  
For example, 64 KB.

Alan Stern

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#1540884

On Mon 12-12-16 16:12:16, Alan Stern wrote:
> On Mon, 12 Dec 2016, Michal Hocko wrote:
> 
> > On Mon 12-12-16 21:32:35, Andrey Konovalov wrote:
> > > On Mon, Dec 12, 2016 at 9:31 PM, Andrey Konovalov <andreyknvl@google.com> wrote:
> > > > Hi!
> > > >
> > > > While running the syzkaller fuzzer I've got the following error report.
> > > >
> > > > The issue is that the len argument is not checked for being too big.
> > 
> > Well, the value is checked in kmalloc_slab. There is a discrepancy
> > though. While the page allocator enforces order < MAX_ORDER, slab
> > allocators enforce size <= KMALLOC_MAX_SIZE and
> > 
> > KMALLOC_MAX_SIZE is 1UL << (MAX_ORDER + PAGE_SHIFT) for SLUB which is
> > what I expect you have or 1UL << (MAX_ORDER + PAGE_SHIFT - 1) for SLAB
> > on most archs.  This means that KMALLOC_MAX_SIZE might be MAX_ORDER for
> > SLUB which would explain your warning.
> > 
> > Let's CC Christoph, this is nothing really new, but I suspect that SLUB
> > should change the maximum size to something like SLAB does.
> > 
> > That being said, what ep_write_iter does sounds quite stupit. It just
> > allocates a large continuous buffer which seems to be under user
> > control...  Aka no good! It should do that per pages or something like
> > that. Something worth fixing
> 
> It's not important enough to make the driver do all this work.  If
> users want to send large amounts of data, they can send it a page at a
> time (or something like that).

Is it really necessary to allocate the full iov_iter_count? Why cannot
we process the from buffer one page at a time?

> If you really want to prevent the driver from attempting to allocate a
> large buffer, all that's needed is an upper limit on the total size.  
> For example, 64 KB.

Well, my point was that it is not really hard to imagine to deplete
larger contiguous memory blocks (say PAGE_ALLOC_COSTLY_ORDER). Those are
still causing the OOM killer and chances are that a controlled flood of
these requests could completely DoS the system.
-- 
Michal Hocko
SUSE Labs

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#1541068

On Tue, 13 Dec 2016, Michal Hocko wrote:

> > > That being said, what ep_write_iter does sounds quite stupit. It just
> > > allocates a large continuous buffer which seems to be under user
> > > control...  Aka no good! It should do that per pages or something like
> > > that. Something worth fixing
> > 
> > It's not important enough to make the driver do all this work.  If
> > users want to send large amounts of data, they can send it a page at a
> > time (or something like that).
> 
> Is it really necessary to allocate the full iov_iter_count? Why cannot
> we process the from buffer one page at a time?

We could (although one page is really too small -- USB 3.1 can transfer
800 KB per ms so we ought to handle at least 128 KB at a time).  But
turn the argument around: If the user wants to transfer that much data,
why can't he _submit_ it one page at a time?

> > If you really want to prevent the driver from attempting to allocate a
> > large buffer, all that's needed is an upper limit on the total size.  
> > For example, 64 KB.
> 
> Well, my point was that it is not really hard to imagine to deplete
> larger contiguous memory blocks (say PAGE_ALLOC_COSTLY_ORDER). Those are
> still causing the OOM killer and chances are that a controlled flood of
> these requests could completely DoS the system.

Putting a limit on the total size of a single transfer would prevent 
this.

Alan Stern

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#1541784

On Tue 13-12-16 08:33:34, Alan Stern wrote:
> On Tue, 13 Dec 2016, Michal Hocko wrote:
> 
> > > > That being said, what ep_write_iter does sounds quite stupit. It just
> > > > allocates a large continuous buffer which seems to be under user
> > > > control...  Aka no good! It should do that per pages or something like
> > > > that. Something worth fixing
> > > 
> > > It's not important enough to make the driver do all this work.  If
> > > users want to send large amounts of data, they can send it a page at a
> > > time (or something like that).
> > 
> > Is it really necessary to allocate the full iov_iter_count? Why cannot
> > we process the from buffer one page at a time?
> 
> We could (although one page is really too small -- USB 3.1 can transfer
> 800 KB per ms so we ought to handle at least 128 KB at a time).

Is there any problem to submit larger transfers without having the
buffer physically contiguous?

> But
> turn the argument around: If the user wants to transfer that much data,
> why can't he _submit_ it one page at a time?

Not sure I understand.
 
> > > If you really want to prevent the driver from attempting to allocate a
> > > large buffer, all that's needed is an upper limit on the total size.  
> > > For example, 64 KB.
> > 
> > Well, my point was that it is not really hard to imagine to deplete
> > larger contiguous memory blocks (say PAGE_ALLOC_COSTLY_ORDER). Those are
> > still causing the OOM killer and chances are that a controlled flood of
> > these requests could completely DoS the system.
> 
> Putting a limit on the total size of a single transfer would prevent 
> this.

Dunno, putting a limit to the user visible interface sounds wrong to me.
-- 
Michal Hocko
SUSE Labs

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#1542016

On Wed 14-12-16 11:13:11, Alan Stern wrote:
> On Wed, 14 Dec 2016, Michal Hocko wrote:
> 
> > On Tue 13-12-16 08:33:34, Alan Stern wrote:
> > > On Tue, 13 Dec 2016, Michal Hocko wrote:
[...]
> > > > Well, my point was that it is not really hard to imagine to deplete
> > > > larger contiguous memory blocks (say PAGE_ALLOC_COSTLY_ORDER). Those are
> > > > still causing the OOM killer and chances are that a controlled flood of
> > > > these requests could completely DoS the system.
> > > 
> > > Putting a limit on the total size of a single transfer would prevent 
> > > this.
> > 
> > Dunno, putting a limit to the user visible interface sounds wrong to me.
> 
> In practice, I think the data transfer sizes tend to be not very large.  
> But I could be wrong about that.

That is one part the other is whether a malicious user can abuse this to
DoS the kernel which is the point I am trying to make here. Depleting
non-costly high orders can be quite dangerious so allowing a free ticket
to them to arbitrary user in an arbitrary amount is definitely not good.
-- 
Michal Hocko
SUSE Labs

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#1542034

On Wed, 14 Dec 2016, Michal Hocko wrote:

> On Tue 13-12-16 08:33:34, Alan Stern wrote:
> > On Tue, 13 Dec 2016, Michal Hocko wrote:
> > 
> > > > > That being said, what ep_write_iter does sounds quite stupit. It just
> > > > > allocates a large continuous buffer which seems to be under user
> > > > > control...  Aka no good! It should do that per pages or something like
> > > > > that. Something worth fixing
> > > > 
> > > > It's not important enough to make the driver do all this work.  If
> > > > users want to send large amounts of data, they can send it a page at a
> > > > time (or something like that).
> > > 
> > > Is it really necessary to allocate the full iov_iter_count? Why cannot
> > > we process the from buffer one page at a time?
> > 
> > We could (although one page is really too small -- USB 3.1 can transfer
> > 800 KB per ms so we ought to handle at least 128 KB at a time).
> 
> Is there any problem to submit larger transfers without having the
> buffer physically contiguous?

Async I/O would be rather awkward; it would have to use a work queue
routine.  But it could be done.

And we would still end up allocating the same total space (more
actually, because we would need to store the scatter-gather table too).  
It just wouldn't be contiguous.

> > But
> > turn the argument around: If the user wants to transfer that much data,
> > why can't he _submit_ it one page at a time?
> 
> Not sure I understand.
>  
> > > > If you really want to prevent the driver from attempting to allocate a
> > > > large buffer, all that's needed is an upper limit on the total size.  
> > > > For example, 64 KB.
> > > 
> > > Well, my point was that it is not really hard to imagine to deplete
> > > larger contiguous memory blocks (say PAGE_ALLOC_COSTLY_ORDER). Those are
> > > still causing the OOM killer and chances are that a controlled flood of
> > > these requests could completely DoS the system.
> > 
> > Putting a limit on the total size of a single transfer would prevent 
> > this.
> 
> Dunno, putting a limit to the user visible interface sounds wrong to me.

In practice, I think the data transfer sizes tend to be not very large.  
But I could be wrong about that.

Alan Stern

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