Subject: Re: linux-next: Tree for May 16 (security/landlock/ruleset.c)

From: Kees Cook <[email-masked]>
Date: Mon, 19 May 2025 12:15:30 -0700

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/[email-masked]/
> > > 
> > > 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 <[email-masked]>
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 <[email-masked]>
Closes: https://lore.kernel.org/all/[email-masked]/
Reported-by: Randy Dunlap <[email-masked]>
Closes: https://lore.kernel.org/all/[email-masked]/
Signed-off-by: Kees Cook <[email-masked]>
---
Cc: "Mickaël Salaün" <[email-masked]>
Cc: Thomas Gleixner <[email-masked]>
Cc: Ingo Molnar <[email-masked]>
Cc: Borislav Petkov <[email-masked]>
Cc: Dave Hansen <[email-masked]>
Cc: <[email-masked]>
Cc: "H. Peter Anvin" <[email-masked]>
Cc: Andy Shevchenko <[email-masked]>
Cc: Uros Bizjak <[email-masked]>
Cc: <[email-masked]>
---
 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