Re: [PATCH] x86: Use __builtin_object_size to validate the buffersize for copy_from_user

[Ingo Molnar]

* Arjan van de Ven <arjan@xxxxxxxxxxxxx> wrote:

> On Sat, 26 Sep 2009 14:41:51 +0200
> Ingo Molnar <mingo@xxxxxxx> wrote:
> 
> > 
> > * Arjan van de Ven <arjan@xxxxxxxxxxxxx> wrote:
> > 
> > > From 524a1da3c45683cec77480acc6cab1d33ae8d5cb Mon Sep 17 00:00:00
> > > 2001 From: Arjan van de Ven <arjan@xxxxxxxxxxxxxxx>
> > > 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@xxxxxxxxxxxxxxx>
> > > Reviewed-by: Ingo Molnar <mingo@xxxxxxx>
> > > ---
> > >  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.

That's pretty convincing.

Linus, any objections?

	Ingo
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