fs: out of bounds on stack in iov_iter_advance

[Sasha Levin <sasha.levin@oracle.com>]

Hi all,

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] ==================================================================


Thanks,
Sasha