Re: fs: out of bounds on stack in iov_iter_advance

[Chuck Ebbert <cebbert.lkml@gmail.com>]

On Wed, 12 Aug 2015 10:13:24 -0400
Sasha Levin <sasha.levin@oracle.com> wrote:

> While fuzzing with trinity inside a KVM tools guest running -next I've stumbled on the following:
> 
> [64092.216447] ==================================================================
> [64092.217840] BUG: KASan: out of bounds on stack in iov_iter_advance+0x3b7/0x480 at addr ffff88040506fd48
> [64092.219314] Read of size 8 by task trinity-c194/11387
> [64092.220114] page:ffffea0010141bc0 count:0 mapcount:0 mapping:          (null) index:0x2
> [64092.221354] flags: 0x46fffff80000000()
> [64092.221998] page dumped because: kasan: bad access detected
> [64092.222879] CPU: 4 PID: 11387 Comm: trinity-c194 Not tainted 4.2.0-rc6-next-20150810-sasha-00040-g12ad0db3-dirty #2427
> [64092.224537]  ffff88040506fd30 ffff88040506fa88 ffffffff9ce7763b ffff88040506fb10
> [64092.225763]  ffff88040506fb00 ffffffff9376b1be 0000000000000000 ffff880270108600
> [64092.226992]  0000000000000282 0000000000000000 0000000000000000 0000000000000000
> [64092.228221] Call Trace:
> [64092.228679] dump_stack (lib/dump_stack.c:52)
> [64092.231252] kasan_report_error (mm/kasan/report.c:132 mm/kasan/report.c:193)
> [64092.232219] __asan_report_load8_noabort (mm/kasan/report.c:251)
> [64092.234167] iov_iter_advance (lib/iov_iter.c:511)
> [64092.235105] generic_file_read_iter (mm/filemap.c:1743)
> [64092.241532] blkdev_read_iter (fs/block_dev.c:1649)
> [64092.242448] __vfs_read (fs/read_write.c:423 fs/read_write.c:434)
> [64092.246949] vfs_read (fs/read_write.c:454)
> [64092.247743] SyS_pread64 (fs/read_write.c:607 fs/read_write.c:594)
> [64092.250445] entry_SYSCALL_64_fastpath (arch/x86/entry/entry_64.S:186)
> [64092.251440] Memory state around the buggy address:
> [64092.252221]  ffff88040506fc00: 00 00 00 f1 f1 f1 f1 00 00 00 00 00 f4 f4 f4 f3
> [64092.253340]  ffff88040506fc80: f3 f3 f3 00 00 00 00 00 00 00 00 00 00 00 00 00
> [64092.254456] >ffff88040506fd00: 00 00 f1 f1 f1 f1 00 00 f4 f4 f2 f2 f2 f2 00 00
> [64092.255566]                                               ^
> [64092.256432]  ffff88040506fd80: 00 00 00 f4 f4 f4 f2 f2 f2 f2 00 00 00 00 00 f4
> [64092.257557]  ffff88040506fe00: f4 f4 f3 f3 f3 f3 00 00 00 00 00 00 00 00 00 00
> [64092.258684] ==================================================================
> 

I tried to debug this but kasan doesn't print much useful information
for stack out of bounds access. It shows the address that's being
accessed but it doesn't show the value of the boundary that was
exceeded. And the stack dump doesn't show any addresses either - just
contents. It would be nice to see a full stack frame dump showing
where all the parent frames are too. Also too the file and line number
(lib/iov_iter.c:511) are completely useless because of inlining,
though that's not kasan's fault.