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Message-ID: <20250520.uof4li6vac3I@digikod.net>
Date: Tue, 20 May 2025 16:45:19 +0200
From: Mickaël Salaün <mic@...ikod.net>
To: Kees Cook <kees@...nel.org>
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 12:15:30PM -0700, Kees Cook wrote:
> 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.
It works for me but I couldn't reproduce the issue. I tried with
CONFIG_PROFILE_ALL_BRANCHES=y and CONFIG_FORTIFY_SOURCE=n but it always
works without a warning. I'm using GCC 15. Is it specific to a version
of GCC?
>
> 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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