Messages in this thread | | | Date | Sat, 26 Sep 2009 16:03:43 +0200 | From | Arjan van de Ven <> | Subject | Re: [PATCH] x86: Use __builtin_object_size to validate the buffer size for copy_from_user |
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On Sat, 26 Sep 2009 14:41:51 +0200 Ingo Molnar <mingo@elte.hu> wrote:
> > * Arjan van de Ven <arjan@infradead.org> wrote: > > > From 524a1da3c45683cec77480acc6cab1d33ae8d5cb Mon Sep 17 00:00:00 > > 2001 From: Arjan van de Ven <arjan@linux.intel.com> > > Date: Sat, 26 Sep 2009 12:36:21 +0200 > > Subject: [PATCH] x86: Use __builtin_object_size to validate the > > buffer size for copy_from_user > > > > gcc (4.x) supports the __builtin_object_size() builtin, which > > reports the size of an object that a pointer point to, when known > > at compile time. If the buffer size is not known at compile time, a > > constant -1 is returned. > > > > This patch uses this feature to add a sanity check to > > copy_from_user(); if the target buffer is known to be smaller than > > the copy size, the copy is aborted and a WARNing is emitted in > > memory debug mode. > > > > These extra checks compile away when the object size is not known, > > or if both the buffer size and the copy length are constants. > > > > Signed-off-by: Arjan van de Ven <arjan@linux.intel.com> > > Reviewed-by: Ingo Molnar <mingo@elte.hu> > > --- > > arch/x86/include/asm/uaccess_32.h | 19 ++++++++++++++++++- > > arch/x86/include/asm/uaccess_64.h | 19 ++++++++++++++++++- > > arch/x86/kernel/x8664_ksyms_64.c | 2 +- > > arch/x86/lib/copy_user_64.S | 4 ++-- > > arch/x86/lib/usercopy_32.c | 4 ++-- > > include/linux/compiler-gcc4.h | 2 ++ > > include/linux/compiler.h | 4 ++++ > > 7 files changed, 47 insertions(+), 7 deletions(-) > > I have tested this on a buffer overflow and it caught it: > > [ 87.056952] ------------[ cut here ]------------ > [ 87.061628] WARNING: > at /home/mingo/linux/arch/x86/include/asm/uaccess_64.h:35 > sys_perf_counter_open+0x112/0x65b() [ 87.072600] Hardware name: > System Product Name [ 87.077072] Buffer overflow detected! > [ 87.080762] Modules linked in: [ 87.083858] Pid: 2670, comm: > exploit Not tainted 2.6.31 #17235 [ 87.089708] Call Trace: > [ 87.092180] [<ffffffff810a3241>] ? > sys_perf_counter_open+0x112/0x65b [ 87.098654] > [<ffffffff8104303c>] warn_slowpath_common+0x77/0xa4 [ 87.104684] > [<ffffffff810430b6>] warn_slowpath_fmt+0x3c/0x3e [ 87.110458] > [<ffffffff810e41c3>] ? putname+0x30/0x39 [ 87.115570] > [<ffffffff810a3241>] sys_perf_counter_open+0x112/0x65b > [ 87.121880] [<ffffffff8105b6df>] ? up_read+0x9/0xb > [ 87.126802] [<ffffffff8100ba6b>] system_call_fastpath+0x16/0x1b > [ 87.132851] ---[ end trace 7469dba2cd3cfea8 ]--- > > > > +static inline unsigned long __must_check copy_from_user(void *to, > > + const void __user *from, > > + unsigned long n) > > +{ > > + int sz = __compiletime_object_size(to); > > + int ret = -EFAULT; > > + > > + if (likely(sz == -1 || sz >= n)) > > + ret = _copy_from_user(to, from, n); > > +#ifdef CONFIG_DEBUG_VM > > + else > > + WARN(1, "Buffer overflow detected!\n"); > > +#endif > > + return ret; > > +} > > This is pretty optimal in the !CONFIG_DEBUG_VM case. Would be nice to > see precisely how optimal - how many new instructions in the default > !CONFIG_DEBUG_VM case? >
a test ->write method:
static ssize_t test_write(struct file *fp, const char __user *buf, size_t len, loff_t *off) { char buffer[10]; int ret;
ret = copy_from_user(&buffer, buf, len);
return ret; }
with the patch turns into
0: 55 push %ebp
* 1: b8 f2 ff ff ff mov $0xfffffff2,%eax 6: 89 e5 mov %esp,%ebp 8: 83 ec 0c sub $0xc,%esp * b: 83 f9 0a cmp $0xa,%ecx * e: 77 08 ja 18 <test_write+0x18> 10: 8d 45 f6 lea -0xa(%ebp),%eax 13: e8 fc ff ff ff call 14 <_copy_from_user> 18: c9 leave 19: c3 ret
while without it gets
0: 55 push %ebp 1: 89 e5 mov %esp,%ebp 3: 83 ec 0c sub $0xc,%esp 6: 8d 45 f6 lea -0xa(%ebp),%eax 9: e8 fc ff ff ff call <copy_from_user> e: c9 leave f: c3 ret
This is for the case where you have a known stack variable, but variable copy size. If you have either an unknown target size and/or a fixed sized copy, the code goes away entirely.
-- Arjan van de Ven Intel Open Source Technology Centre For development, discussion and tips for power savings, visit http://www.lesswatts.org
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