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[PATCH] /dev/mem: remove superfluous error

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  [PATCH] /dev/mem: remove superfluous error Jiri Kosina <jikos@kernel.org> - 2016-07-08 11:50 +0200
    [tip:x86/mm] x86/mm/pat, /dev/mem: Remove superfluous error message tip-bot for Jiri Kosina <tipbot@zytor.com> - 2016-07-08 14:20 +0200

#1439256 — [PATCH] /dev/mem: remove superfluous error

From: Jiri Kosina <jkosina@suse.cz>

Currently it's possible for broken (or malicious) userspace to flood a 
kernel log indefinitely with messages a-la

	Program dmidecode tried to access /dev/mem between f0000->100000

because range_is_allowed() is case of CONFIG_STRICT_DEVMEM being turned on 
dumps this information each and every time devmem_is_allowed() fails.

Reportedly userspace that is able to trigger contignuous flow of these 
messages exists.

It would be possible to rate limit this message, but that'd have a 
questionable value; the administrator wouldn't get information about all 
the failing accessess, so then the information would be both superfluous 
and incomplete at the same time :)

Returning EPERM (which is what is actually happening) is enough indication 
for userspace what has happened; no need to log this particular error as 
some sort of special condition.

Signed-off-by: Jiri Kosina <jkosina@suse.cz>
---
 arch/x86/mm/pat.c  | 5 +----
 drivers/char/mem.c | 6 +-----
 2 files changed, 2 insertions(+), 9 deletions(-)

diff --git a/arch/x86/mm/pat.c b/arch/x86/mm/pat.c
index fb0604f..db00e3e 100644
--- a/arch/x86/mm/pat.c
+++ b/arch/x86/mm/pat.c
@@ -755,11 +755,8 @@ static inline int range_is_allowed(unsigned long pfn, unsigned long size)
 		return 1;
 
 	while (cursor < to) {
-		if (!devmem_is_allowed(pfn)) {
-			pr_info("x86/PAT: Program %s tried to access /dev/mem between [mem %#010Lx-%#010Lx], PAT prevents it\n",
-				current->comm, from, to - 1);
+		if (!devmem_is_allowed(pfn))
 			return 0;
-		}
 		cursor += PAGE_SIZE;
 		pfn++;
 	}
diff --git a/drivers/char/mem.c b/drivers/char/mem.c
index 71025c2..d633974 100644
--- a/drivers/char/mem.c
+++ b/drivers/char/mem.c
@@ -66,12 +66,8 @@ static inline int range_is_allowed(unsigned long pfn, unsigned long size)
 	u64 cursor = from;
 
 	while (cursor < to) {
-		if (!devmem_is_allowed(pfn)) {
-			printk(KERN_INFO
-		"Program %s tried to access /dev/mem between %Lx->%Lx.\n",
-				current->comm, from, to);
+		if (!devmem_is_allowed(pfn))
 			return 0;
-		}
 		cursor += PAGE_SIZE;
 		pfn++;
 	}

-- 
Jiri Kosina
SUSE Labs

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#1439393 — [tip:x86/mm] x86/mm/pat, /dev/mem: Remove superfluous error message

Commit-ID:  39380b80d72723282f0ea1d1bbf2294eae45013e
Gitweb:     http://git.kernel.org/tip/39380b80d72723282f0ea1d1bbf2294eae45013e
Author:     Jiri Kosina <jkosina@suse.cz>
AuthorDate: Fri, 8 Jul 2016 11:38:28 +0200
Committer:  Ingo Molnar <mingo@kernel.org>
CommitDate: Fri, 8 Jul 2016 11:52:58 +0200

x86/mm/pat, /dev/mem: Remove superfluous error message

Currently it's possible for broken (or malicious) userspace to flood a
kernel log indefinitely with messages a-la

	Program dmidecode tried to access /dev/mem between f0000->100000

because range_is_allowed() is case of CONFIG_STRICT_DEVMEM being turned on
dumps this information each and every time devmem_is_allowed() fails.

Reportedly userspace that is able to trigger contignuous flow of these
messages exists.

It would be possible to rate limit this message, but that'd have a
questionable value; the administrator wouldn't get information about all
the failing accessess, so then the information would be both superfluous
and incomplete at the same time :)

Returning EPERM (which is what is actually happening) is enough indication
for userspace what has happened; no need to log this particular error as
some sort of special condition.

Signed-off-by: Jiri Kosina <jkosina@suse.cz>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Luis R. Rodriguez <mcgrof@suse.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Toshi Kani <toshi.kani@hp.com>
Link: http://lkml.kernel.org/r/alpine.LNX.2.00.1607081137020.24757@cbobk.fhfr.pm
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 arch/x86/mm/pat.c  | 5 +----
 drivers/char/mem.c | 6 +-----
 2 files changed, 2 insertions(+), 9 deletions(-)

diff --git a/arch/x86/mm/pat.c b/arch/x86/mm/pat.c
index fb0604f..db00e3e 100644
--- a/arch/x86/mm/pat.c
+++ b/arch/x86/mm/pat.c
@@ -755,11 +755,8 @@ static inline int range_is_allowed(unsigned long pfn, unsigned long size)
 		return 1;
 
 	while (cursor < to) {
-		if (!devmem_is_allowed(pfn)) {
-			pr_info("x86/PAT: Program %s tried to access /dev/mem between [mem %#010Lx-%#010Lx], PAT prevents it\n",
-				current->comm, from, to - 1);
+		if (!devmem_is_allowed(pfn))
 			return 0;
-		}
 		cursor += PAGE_SIZE;
 		pfn++;
 	}
diff --git a/drivers/char/mem.c b/drivers/char/mem.c
index 71025c2..d633974 100644
--- a/drivers/char/mem.c
+++ b/drivers/char/mem.c
@@ -66,12 +66,8 @@ static inline int range_is_allowed(unsigned long pfn, unsigned long size)
 	u64 cursor = from;
 
 	while (cursor < to) {
-		if (!devmem_is_allowed(pfn)) {
-			printk(KERN_INFO
-		"Program %s tried to access /dev/mem between %Lx->%Lx.\n",
-				current->comm, from, to);
+		if (!devmem_is_allowed(pfn))
 			return 0;
-		}
 		cursor += PAGE_SIZE;
 		pfn++;
 	}

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