On Fri, Jul 14, 2017 at 12:05:02PM -0700, Linus Torvalds wrote:
> On Fri, Jul 14, 2017 at 7:25 AM, Dave Jones <da...@codemonkey.org.uk> wrote:
> > On Thu, Jul 13, 2017 at 05:16:24PM -0400, Anna Schumaker wrote:
> > >
> > > git://git.linux-nfs.org/projects/anna/linux-nfs.git
> > tags/nfs-for-4.13-1
> >
> > Since this landed, I'm seeing this during boot..
> >
> > ==================================================================
> > BUG: KASAN: global-out-of-bounds in strscpy+0x4a/0x230
> > Read of size 8 at addr ffffffffb4eeaf20 by task nfsd/688
>
> Is KASAN aware that strscpy() does the word-at-a-time optimistic reads
> of the sources?
>
> The problem may be that the source is initialized from the global
> string "nfsd", and KASAN may be unhappy abotu the fact that we read 8
> bytes from a 5-byte string (four plus NUL) as we do the word-at-a-time
> strscpy..
>
> That said, we do check the size first (because we also *write* 8 bytes
> at a time), so maybe KASAN shouldn't even need to care.
>
> Hmm. it really looks to me like this is actually a compiler bug (I'm
> using current gcc in F26, which is gcc-7.1.1 - I'm assuming DaveJ is
> the same).
Debian's 6.4.0
> This is the source code in __ip_map_lookup:
>
> struct ip_map ip;
> .....
> strcpy(ip.m_class, class);
>
> and "m_class" is 8 bytes in size:
>
> struct ip_map {
> ...
> char m_class[8]; /* e.g. "nfsd" */
> ...
>
> yet when I look at the generated code for __ip_map_lookup, I see
>
> movl $32, %edx #,
> movq %r13, %rsi # class,
> leaq 48(%rax), %rdi #, tmp126
> call strscpy #
>
> what's the bug here? Look at that third argument - %rdx. It is
> initialized to 32.
>
> WTF?
>
> The code to turn "strcpy()" into "strscpy()" should pick the *smaller*
> of the two object sizes as the size argument. How the hell is that
> size argument 32?
>
> Am I missing something? DaveJ, do you see the same?
My compiler seems to have replaced the call with an inlined copy afaics.
0000000000000be0 <__ip_map_lookup>:
{
be0: e8 00 00 00 00 callq be5 <__ip_map_lookup+0x5>
be5: 55 push %rbp
be6: 48 b8 00 00 00 00 00 movabs $0xdffffc0000000000,%rax
bed: fc ff df
bf0: 48 89 e5 mov %rsp,%rbp
bf3: 41 57 push %r15
bf5: 41 56 push %r14
bf7: 4c 8d 32 lea (%rdx),%r14
if (strscpy(p, q, p_size < q_size ? p_size : q_size) < 0)
bfa: ba 20 00 00 00 mov $0x20,%edx
bff: 41 55 push %r13
c01: 4c 8d 2e lea (%rsi),%r13
c04: 41 54 push %r12
c06: 53 push %rbx
c07: 48 8d 1f lea (%rdi),%rbx
c0a: 48 8d a4 24 60 ff ff lea -0xa0(%rsp),%rsp
c11: ff
c12: 49 89 e4 mov %rsp,%r12
c15: 49 c1 ec 03 shr $0x3,%r12
c19: 48 c7 04 24 b3 8a b5 movq $0x41b58ab3,(%rsp)
c20: 41
c21: 48 c7 44 24 08 00 00 movq $0x0,0x8(%rsp)
c28: 00 00
c2a: 48 c7 44 24 10 00 00 movq $0x0,0x10(%rsp)
c31: 00 00
c33: 48 8d 7c 24 50 lea 0x50(%rsp),%rdi
c38: 4d 8d 24 04 lea (%r12,%rax,1),%r12
c3c: 41 c7 04 24 f1 f1 f1 movl $0xf1f1f1f1,(%r12)
c43: f1
c44: 41 c7 44 24 0c 00 00 movl $0xf4f40000,0xc(%r12)
c4b: f4 f4
c4d: 65 48 8b 04 25 28 00 mov %gs:0x28,%rax
c54: 00 00
c56: 48 89 84 24 98 00 00 mov %rax,0x98(%rsp)
c5d: 00
c5e: 31 c0 xor %eax,%eax
c60: e8 00 00 00 00 callq c65 <__ip_map_lookup+0x85>
c65: 48 85 c0 test %rax,%rax
c68: 0f 88 a0 00 00 00 js d0e <__ip_map_lookup+0x12e>
ip.m_addr = *addr;
c6e: be 10 00 00 00 mov $0x10,%esi
c73: 49 8d 3e lea (%r14),%rdi
c76: e8 00 00 00 00 callq c7b <__ip_map_lookup+0x9b>
c7b: 49 8b 56 08 mov 0x8(%r14),%rdx
But that mov $0x20,%edx looks like it might be the same value we're talking
about.
Dave
