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Message-ID: <202505191212.61EE1AE80@keescook>
Date: Mon, 19 May 2025 12:15:30 -0700
From: Kees Cook <kees@...nel.org>
To: Mickaël Salaün <mic@...ikod.net>
Cc: Randy Dunlap <rdunlap@...radead.org>,
Steven Rostedt <rostedt@...dmis.org>,
Stephen Rothwell <sfr@...b.auug.org.au>,
Linux Next Mailing List <linux-next@...r.kernel.org>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
linux-security-module@...r.kernel.org,
Günther Noack <gnoack@...gle.com>
Subject: Re: linux-next: Tree for May 16 (security/landlock/ruleset.c)
On Mon, May 19, 2025 at 08:41:17PM +0200, Mickaël Salaün wrote:
> On Mon, May 19, 2025 at 11:19:53AM -0700, Kees Cook wrote:
> > On Mon, May 19, 2025 at 05:29:30PM +0200, Mickaël Salaün wrote:
> > > On Fri, May 16, 2025 at 07:54:14PM -0700, Randy Dunlap wrote:
> > > >
> > > >
> > > > On 5/16/25 3:24 AM, Stephen Rothwell wrote:
> > > > > Hi all,
> > > > >
> > > > > Changes since 20250515:
> > >
> > > Thanks for the report.
> > >
> > > It is the same warning as reported here:
> > > https://lore.kernel.org/all/202501040747.S3LYfvYq-lkp@intel.com/
> > >
> > > I don't know what the actual issue is though.
> > >
> > > >
> > > > on i386:
> > > >
> > > > In file included from ../arch/x86/include/asm/string.h:3,
> > > > from ../include/linux/string.h:65,
> > > > from ../include/linux/bitmap.h:13,
> > > > from ../include/linux/cpumask.h:12,
> > > > from ../include/linux/smp.h:13,
> > > > from ../include/linux/lockdep.h:14,
> > > > from ../security/landlock/ruleset.c:16:
> > > > ../security/landlock/ruleset.c: In function 'create_rule':
> > > > ../arch/x86/include/asm/string_32.h:150:25: warning: '__builtin_memcpy' accessing 4294967295 bytes at offsets 20 and 0 overlaps 6442450943 bytes at offset -2147483648 [-Wrestrict]
> > > > 150 | #define memcpy(t, f, n) __builtin_memcpy(t, f, n)
> > > > | ^~~~~~~~~~~~~~~~~~~~~~~~~
> > > > ../security/landlock/ruleset.c:137:9: note: in expansion of macro 'memcpy'
> > > > 137 | memcpy(new_rule->layers, layers,
> > > > | ^~~~~~
> > > >
> > > >
> > > > Full randconfig file is attached.
> >
> > The trigger appears to be CONFIG_PROFILE_ALL_BRANCHES, and GCC getting
> > tricked into thinking check_mul_overflow() returns true:
> >
> > In file included from ../arch/x86/include/asm/string.h:3,
> > from ../include/linux/string.h:65,
> > from ../include/linux/bitmap.h:13,
> > from ../include/linux/cpumask.h:12,
> > from ../include/linux/smp.h:13,
> > from ../include/linux/lockdep.h:14,
> > from ../security/landlock/ruleset.c:16:
> > ../security/landlock/ruleset.c: In function 'create_rule':
> > ../arch/x86/include/asm/string_32.h:150:25: warning: '__builtin_memcpy' accessing 4294967295 bytes at offsets 0 and 0 overlaps 6442450943 bytes at offset -2147483648 [-Wrestrict]
> > 150 | #define memcpy(t, f, n) __builtin_memcpy(t, f, n)
> > | ^~~~~~~~~~~~~~~~~~~~~~~~~
> > ../security/landlock/ruleset.c:137:9: note: in expansion of macro 'memcpy'
> > 137 | memcpy(new_rule->layers, layers,
> > | ^~~~~~
> > 'create_rule': event 1
> > ../include/linux/compiler.h:69:46:
> > 68 | (cond) ? \
> > | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > 69 | (__if_trace.miss_hit[1]++,1) : \
> > | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~
> > | |
> > | (1) when the condition is evaluated to true
> > 70 | (__if_trace.miss_hit[0]++,0); \
> > | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> > ../include/linux/compiler.h:57:69: note: in expansion of macro '__trace_if_value'
> > 57 | #define __trace_if_var(cond) (__builtin_constant_p(cond) ? (cond) : __trace_if_value(cond))
> > | ^~~~~~~~~~~~~~~~
> > ../include/linux/compiler.h:55:28: note: in expansion of macro '__trace_if_var'
> > 55 | #define if(cond, ...) if ( __trace_if_var( !!(cond , ## __VA_ARGS__) ) )
> > | ^~~~~~~~~~~~~~
> > ../include/linux/overflow.h:270:9: note: in expansion of macro 'if'
> > 270 | if (check_mul_overflow(factor1, factor2, &bytes))
> > | ^~
> > 'create_rule': event 2
> > ../arch/x86/include/asm/string_32.h:150:25:
> > 150 | #define memcpy(t, f, n) __builtin_memcpy(t, f, n)
> > | ^~~~~~~~~~~~~~~~~~~~~~~~~
> > | |
> > | (2) out of array bounds here
> > ../security/landlock/ruleset.c:137:9: note: in expansion of macro 'memcpy'
> > 137 | memcpy(new_rule->layers, layers,
> > | ^~~~~~
> > make[1]: Leaving directory '/srv/code/gcc-bug'
>
> That's interesting...
>
> >
> >
> > I'll take a look at ways to make either the overflow macros or memcpy
> > robust against this kind of weirdness...
>
> Thanks!
I'm doing some build testing, but the below patch makes GCC happy.
Alternatively we could make CONFIG_PROFILE_ALL_BRANCHES=y depend on
CONFIG_FORTIFY_SOURCE=y ...
>From 6fbf66fdfd0a7dac809b77faafdd72c60112bb8d Mon Sep 17 00:00:00 2001
From: Kees Cook <kees@...nel.org>
Date: Mon, 19 May 2025 11:52:06 -0700
Subject: [PATCH] string.h: Provide basic sanity checks for fallback memcpy()
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit
Instead of defining memcpy() in terms of __builtin_memcpy() deep
in arch/x86/include/asm/string_32.h, notice that it is needed up in
the general string.h, as done with other common C String APIs. This
allows us to add basic sanity checking for pathological "size"
arguments to memcpy(). Besides the run-time checking benefit, this
avoids GCC trying to be very smart about value range tracking[1] when
CONFIG_PROFILE_ALL_BRANCHES=y but FORTIFY_SOURCE=n.
Link: https://lore.kernel.org/all/202505191117.C094A90F88@keescook/ [1]
Reported-by: kernel test robot <lkp@...el.com>
Closes: https://lore.kernel.org/all/202501040747.S3LYfvYq-lkp@intel.com/
Reported-by: Randy Dunlap <rdunlap@...radead.org>
Closes: https://lore.kernel.org/all/e3754f69-1dea-4542-8de0-a567a14fb95b@infradead.org/
Signed-off-by: Kees Cook <kees@...nel.org>
---
Cc: "Mickaël Salaün" <mic@...ikod.net>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Ingo Molnar <mingo@...hat.com>
Cc: Borislav Petkov <bp@...en8.de>
Cc: Dave Hansen <dave.hansen@...ux.intel.com>
Cc: <x86@...nel.org>
Cc: "H. Peter Anvin" <hpa@...or.com>
Cc: Andy Shevchenko <andy@...nel.org>
Cc: Uros Bizjak <ubizjak@...il.com>
Cc: <linux-hardening@...r.kernel.org>
---
arch/x86/include/asm/string_32.h | 6 ------
include/linux/string.h | 13 +++++++++++++
2 files changed, 13 insertions(+), 6 deletions(-)
diff --git a/arch/x86/include/asm/string_32.h b/arch/x86/include/asm/string_32.h
index e9cce169bb4c..74397c95fa37 100644
--- a/arch/x86/include/asm/string_32.h
+++ b/arch/x86/include/asm/string_32.h
@@ -145,12 +145,6 @@ static __always_inline void *__constant_memcpy(void *to, const void *from,
#define __HAVE_ARCH_MEMCPY
extern void *memcpy(void *, const void *, size_t);
-#ifndef CONFIG_FORTIFY_SOURCE
-
-#define memcpy(t, f, n) __builtin_memcpy(t, f, n)
-
-#endif /* !CONFIG_FORTIFY_SOURCE */
-
#define __HAVE_ARCH_MEMMOVE
void *memmove(void *dest, const void *src, size_t n);
diff --git a/include/linux/string.h b/include/linux/string.h
index 01621ad0f598..ffcee31a14f9 100644
--- a/include/linux/string.h
+++ b/include/linux/string.h
@@ -3,6 +3,7 @@
#define _LINUX_STRING_H_
#include <linux/args.h>
+#include <linux/bug.h>
#include <linux/array_size.h>
#include <linux/cleanup.h> /* for DEFINE_FREE() */
#include <linux/compiler.h> /* for inline */
@@ -390,7 +391,19 @@ static inline const char *kbasename(const char *path)
#if !defined(__NO_FORTIFY) && defined(__OPTIMIZE__) && defined(CONFIG_FORTIFY_SOURCE)
#include <linux/fortify-string.h>
+#else
+/* Basic sanity checking even without FORTIFY_SOURCE */
+# ifndef __HAVE_ARCH_MEMCPY
+# define memcpy(t, f, n) \
+ do { \
+ typeof(n) __n = (n); \
+ /* Skip impossible sizes. */ \
+ if (!WARN_ON(__n < 0 || __n == SIZE_MAX)) \
+ __builtin_memcpy(t, f, __n); \
+ } while (0)
+# endif
#endif
+
#ifndef unsafe_memcpy
#define unsafe_memcpy(dst, src, bytes, justification) \
memcpy(dst, src, bytes)
--
2.34.1
--
Kees Cook
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