Re: [devkmsg_write] BUG: KASAN: slab-out-of-bounds in copyin+0xea/0x170

[Andrey Ryabinin]

On 11/07/2017 12:39 PM, Fengguang Wu wrote:
> Hello,
> 
> FYI this happens in v4.14-rc8 -- it's not necessarily a new bug.
> 
> [   22.184920] Freeing unused kernel memory: 824K
> [   22.199198] Freeing unused kernel memory: 1436K
> [   22.228460] x86/mm: Checked W+X mappings: passed, no W+X pages found.
> [   22.230474] rodata_test: all tests were successful
> [   22.254830] ==================================================================
> [   22.257125] BUG: KASAN: slab-out-of-bounds in copyin+0xea/0x170
> [   22.258648] Write of size 26 at addr ffff880013432540 by task init/1
> [   22.260272] 


It says that we write 26 bytes starting from ffff880013432540.

But according to the dump of shadow memory:

[   22.297667] >ffff880013432500: fc fc fb fb fb fb fc fc 00 00 00 03 fc fc fb fb
[   22.298581]                                                     ^


we have allocated 27 bytes at address stating from ffff880013432540.
So this looks like a false positive. I've seen reports like this before:
https://lkml.kernel.org/r/<5163c242-4bbe-7fec-ab58-9c360abf0afd@xxxxxxxxxxxxx>
https://lkml.kernel.org/r/<899f0c39-81b5-5d02-5ced-937884d22c89@xxxxxxxxx>

This happens only on old gcc versions, in your case it's 4.6:
[    0.000000] Linux version 4.14.0-rc8 (kbuild@lkp-wsx01) (gcc version 4.6.4 (Debian 4.6.4-7)) #14 SMP PREEMPT Tue Nov 7 06:50:21 CST 2017

Old GCC is misscompiling something in check_memory_region() which cause false-positive like this one.
I never bothered to dig it deeper, because I've never seen this with GCC versions (>= 4.9.2) that actually support KASAN.
It's almost pointless to use KASAN=y kernel built with old GCC. All memory accesses, except manually instrumented with kasan_check_[read|write](),
won't be checked.