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[PATCH 3.18 47/59] arm64: documentation: document tagged pointer stack constraints

From Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Newsgroups linux.kernel
Subject [PATCH 3.18 47/59] arm64: documentation: document tagged pointer stack constraints
Date 2017-05-23 23:10 +0200
Message-ID <tKpe4-rD-77@gated-at.bofh.it> (permalink)
References <tKpe1-rD-7@gated-at.bofh.it>
Organization linux.* mail to news gateway

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3.18-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Kristina Martsenko <kristina.martsenko@arm.com>

commit f0e421b1bf7af97f026e1bb8bfe4c5a7a8c08f42 upstream.

Some kernel features don't currently work if a task puts a non-zero
address tag in its stack pointer, frame pointer, or frame record entries
(FP, LR).

For example, with a tagged stack pointer, the kernel can't deliver
signals to the process, and the task is killed instead. As another
example, with a tagged frame pointer or frame records, perf fails to
generate call graphs or resolve symbols.

For now, just document these limitations, instead of finding and fixing
everything that doesn't work, as it's not known if anyone needs to use
tags in these places anyway.

In addition, as requested by Dave Martin, generalize the limitations
into a general kernel address tag policy, and refactor
tagged-pointers.txt to include it.

Fixes: d50240a5f6ce ("arm64: mm: permit use of tagged pointers at EL0")
Reviewed-by: Dave Martin <Dave.Martin@arm.com>
Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Kristina Martsenko <kristina.martsenko@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 Documentation/arm64/tagged-pointers.txt |   66 +++++++++++++++++++++++---------
 1 file changed, 49 insertions(+), 17 deletions(-)

--- a/Documentation/arm64/tagged-pointers.txt
+++ b/Documentation/arm64/tagged-pointers.txt
@@ -11,24 +11,56 @@ in AArch64 Linux.
 The kernel configures the translation tables so that translations made
 via TTBR0 (i.e. userspace mappings) have the top byte (bits 63:56) of
 the virtual address ignored by the translation hardware. This frees up
-this byte for application use, with the following caveats:
+this byte for application use.
 
-	(1) The kernel requires that all user addresses passed to EL1
-	    are tagged with tag 0x00. This means that any syscall
-	    parameters containing user virtual addresses *must* have
-	    their top byte cleared before trapping to the kernel.
-
-	(2) Non-zero tags are not preserved when delivering signals.
-	    This means that signal handlers in applications making use
-	    of tags cannot rely on the tag information for user virtual
-	    addresses being maintained for fields inside siginfo_t.
-	    One exception to this rule is for signals raised in response
-	    to watchpoint debug exceptions, where the tag information
-	    will be preserved.
-
-	(3) Special care should be taken when using tagged pointers,
-	    since it is likely that C compilers will not hazard two
-	    virtual addresses differing only in the upper byte.
+
+Passing tagged addresses to the kernel
+--------------------------------------
+
+All interpretation of userspace memory addresses by the kernel assumes
+an address tag of 0x00.
+
+This includes, but is not limited to, addresses found in:
+
+ - pointer arguments to system calls, including pointers in structures
+   passed to system calls,
+
+ - the stack pointer (sp), e.g. when interpreting it to deliver a
+   signal,
+
+ - the frame pointer (x29) and frame records, e.g. when interpreting
+   them to generate a backtrace or call graph.
+
+Using non-zero address tags in any of these locations may result in an
+error code being returned, a (fatal) signal being raised, or other modes
+of failure.
+
+For these reasons, passing non-zero address tags to the kernel via
+system calls is forbidden, and using a non-zero address tag for sp is
+strongly discouraged.
+
+Programs maintaining a frame pointer and frame records that use non-zero
+address tags may suffer impaired or inaccurate debug and profiling
+visibility.
+
+
+Preserving tags
+---------------
+
+Non-zero tags are not preserved when delivering signals. This means that
+signal handlers in applications making use of tags cannot rely on the
+tag information for user virtual addresses being maintained for fields
+inside siginfo_t. One exception to this rule is for signals raised in
+response to watchpoint debug exceptions, where the tag information will
+be preserved.
 
 The architecture prevents the use of a tagged PC, so the upper byte will
 be set to a sign-extension of bit 55 on exception return.
+
+
+Other considerations
+--------------------
+
+Special care should be taken when using tagged pointers, since it is
+likely that C compilers will not hazard two virtual addresses differing
+only in the upper byte.

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[PATCH 3.18 00/59] 3.18.55-stable review Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-05-23 23:10 +0200
  [PATCH 3.18 31/59] USB: serial: io_ti: fix div-by-zero in set_termios Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-05-23 23:10 +0200
  [PATCH 3.18 18/59] USB: serial: ftdi_sio: fix setting latency for unprivileged users Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-05-23 23:10 +0200
  [PATCH 3.18 52/59] stackprotector: Increase the per-task stack canarys random range from 32 bits to 64 bits on 64-bit platforms Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-05-23 23:10 +0200
  [PATCH 3.18 44/59] ARM: dts: at91: sama5d3_xplained: fix ADC vref Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-05-23 23:10 +0200
  [PATCH 3.18 58/59] drivers: char: mem: Check for address space wraparound with mmap() Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-05-23 23:10 +0200
  [PATCH 3.18 27/59] USB: iowarrior: fix info ioctl on big-endian hosts Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-05-23 23:10 +0200
  [PATCH 3.18 48/59] xc2028: Fix use-after-free bug properly Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-05-23 23:10 +0200
  [PATCH 3.18 55/59] tracing/kprobes: Enforce kprobes teardown after testing Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-05-23 23:10 +0200
  [PATCH 3.18 20/59] usb: host: xhci-plat: propagate return value of platform_get_irq() Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-05-23 23:10 +0200
  [PATCH 3.18 05/59] dm btree: fix for dm_btree_find_lowest_key() Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-05-23 23:10 +0200
  [PATCH 3.18 40/59] [media] zr364xx: enforce minimum size when reading header Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-05-23 23:10 +0200
  [PATCH 3.18 24/59] [media] mceusb: fix NULL-deref at probe Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-05-23 23:10 +0200
  [PATCH 3.18 42/59] [media] cx231xx-audio: fix NULL-deref at probe Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-05-23 23:10 +0200
  [PATCH 3.18 09/59] mwifiex: pcie: fix cmd_buf use-after-free in remove/reset Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-05-23 23:10 +0200
  [PATCH 3.18 16/59] iio: dac: ad7303: fix channel description Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-05-23 23:10 +0200
  [PATCH 3.18 11/59] regulator: tps65023: Fix inverted core enable logic. Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-05-23 23:10 +0200
  [PATCH 3.18 34/59] tty: Prevent ldisc drivers from re-using stale tty fields Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-05-23 23:10 +0200
    Re: [PATCH 3.18 34/59] tty: Prevent ldisc drivers from re-using  stale tty fields Alan Cox <gnomes@lxorguk.ukuu.org.uk> - 2017-05-24 15:50 +0200
      Re: [PATCH 3.18 34/59] tty: Prevent ldisc drivers from re-using  stale tty fields Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-05-24 17:10 +0200
  [PATCH 3.18 46/59] arm64: uaccess: ensure extension of access_ok() addr Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-05-23 23:10 +0200
  [PATCH 3.18 30/59] USB: serial: mct_u232: fix big-endian baud-rate handling Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-05-23 23:10 +0200
  [PATCH 3.18 32/59] USB: hub: fix SS hub-descriptor handling Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-05-23 23:10 +0200
  [PATCH 3.18 41/59] [media] cx231xx-cards: fix NULL-deref at probe Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-05-23 23:10 +0200
  [PATCH 3.18 47/59] arm64: documentation: document tagged pointer stack constraints Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-05-23 23:10 +0200
  [PATCH 3.18 13/59] cdc-acm: fix possible invalid access when processing notification Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-05-23 23:10 +0200
  [PATCH 3.18 50/59] metag/uaccess: Fix access_ok() Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-05-23 23:10 +0200
  [PATCH 3.18 54/59] osf_wait4(): fix infoleak Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-05-23 23:10 +0200
  [PATCH 3.18 49/59] mm/huge_memory.c: respect FOLL_FORCE/FOLL_COW for thp Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-05-23 23:10 +0200
  [PATCH 3.18 51/59] metag/uaccess: Check access_ok in strncpy_from_user Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-05-23 23:10 +0200
  [PATCH 3.18 33/59] USB: hub: fix non-SS hub-descriptor handling Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-05-23 23:10 +0200
  [PATCH 3.18 45/59] ARM: dts: at91: sama5d3_xplained: not all ADC channels are available Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-05-23 23:10 +0200
  [PATCH 3.18 28/59] usb: serial: option: add Telit ME910 support Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-05-23 23:10 +0200
  [PATCH 3.18 15/59] of: fdt: add missing allocation-failure check Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-05-23 23:10 +0200
  [PATCH 3.18 26/59] usb: musb: tusb6010_omap: Do not reset the other directions packet size Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-05-23 23:10 +0200
  [PATCH 3.18 23/59] [media] usbvision: fix NULL-deref at probe Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-05-23 23:10 +0200
  [PATCH 3.18 53/59] uwb: fix device quirk on big-endian hosts Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-05-23 23:10 +0200
  [PATCH 3.18 56/59] PCI: Fix pci_mmap_fits() for HAVE_PCI_RESOURCE_TO_USER platforms Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-05-23 23:10 +0200
  [PATCH 3.18 36/59] iio: proximity: as3935: fix as3935_write Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-05-23 23:10 +0200
  [PATCH 3.18 17/59] pid_ns: Sleep in TASK_INTERRUPTIBLE in zap_pid_ns_processes Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-05-23 23:20 +0200
  [PATCH 3.18 06/59] dm bufio: avoid a possible ABBA deadlock Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-05-23 23:20 +0200
  [PATCH 3.18 19/59] USB: serial: ftdi_sio: add Olimex ARM-USB-TINY(H) PIDs Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-05-23 23:20 +0200
  [PATCH 3.18 12/59] ath9k_htc: fix NULL-deref at probe Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-05-23 23:20 +0200
  [PATCH 3.18 22/59] net: irda: irda-usb: fix firmware name on big-endian hosts Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-05-23 23:20 +0200
  [PATCH 3.18 25/59] [media] ttusb2: limit messages to buffer size Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-05-23 23:20 +0200
  [PATCH 3.18 14/59] of: fix sparse warning in of_pci_range_parser_one Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-05-23 23:20 +0200
  [PATCH 3.18 04/59] USB: core: replace %p with %pK Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-05-23 23:20 +0200
  [PATCH 3.18 08/59] dm space map disk: fix some book keeping in the disk space map Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-05-23 23:20 +0200
  [PATCH 3.18 07/59] dm thin metadata: call precommit before saving the roots Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-05-23 23:20 +0200
  [PATCH 3.18 39/59] [media] dib0700: fix NULL-deref at probe Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-05-23 23:20 +0200
  [PATCH 3.18 38/59] [media] s5p-mfc: Fix unbalanced call to clock management Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-05-23 23:20 +0200
  [PATCH 3.18 43/59] powerpc/pseries: Fix of_node_put() underflow during DLPAR remove Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-05-23 23:20 +0200
  [PATCH 3.18 37/59] [media] gspca: konica: add missing endpoint sanity check Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-05-23 23:20 +0200
  [PATCH 3.18 03/59] char: lp: fix possible integer overflow in lp_setup() Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-05-23 23:20 +0200
  [PATCH 3.18 02/59] watchdog: pcwd_usb: fix NULL-deref at probe Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-05-23 23:20 +0200
  [PATCH 3.18 01/59] USB: ene_usb6250: fix DMA to the stack Greg Kroah-Hartman <gregkh@linuxfoundation.org> - 2017-05-23 23:20 +0200
  Re: [PATCH 3.18 00/59] 3.18.55-stable review Guenter Roeck <linux@roeck-us.net> - 2017-05-24 17:50 +0200

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