On 07/29/2016 02:40 AM, Vegard Nossum wrote:
I got a KASAN report of use-after-free:
==================================================================
BUG: KASAN: use-after-free in klist_iter_exit+0x61/0x70 at addr ffff8800b6581508
Read of size 8 by task trinity-c1/315
=============================================================================
BUG kmalloc-32 (Not tainted): kasan: bad access detected
-----------------------------------------------------------------------------
Disabling lock debugging due to kernel taint
INFO: Allocated in disk_seqf_start+0x66/0x110 age=144 cpu=1 pid=315
___slab_alloc+0x4f1/0x520
__slab_alloc.isra.58+0x56/0x80
kmem_cache_alloc_trace+0x260/0x2a0
disk_seqf_start+0x66/0x110
traverse+0x176/0x860
seq_read+0x7e3/0x11a0
proc_reg_read+0xbc/0x180
do_loop_readv_writev+0x134/0x210
do_readv_writev+0x565/0x660
vfs_readv+0x67/0xa0
do_preadv+0x126/0x170
SyS_preadv+0xc/0x10
do_syscall_64+0x1a1/0x460
return_from_SYSCALL_64+0x0/0x6a
INFO: Freed in disk_seqf_stop+0x42/0x50 age=160 cpu=1 pid=315
__slab_free+0x17a/0x2c0
kfree+0x20a/0x220
disk_seqf_stop+0x42/0x50
traverse+0x3b5/0x860
seq_read+0x7e3/0x11a0
proc_reg_read+0xbc/0x180
do_loop_readv_writev+0x134/0x210
do_readv_writev+0x565/0x660
vfs_readv+0x67/0xa0
do_preadv+0x126/0x170
SyS_preadv+0xc/0x10
do_syscall_64+0x1a1/0x460
return_from_SYSCALL_64+0x0/0x6a
CPU: 1 PID: 315 Comm: trinity-c1 Tainted: G B 4.7.0+ #62
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Ubuntu-1.8.2-1ubuntu1 04/01/2014
ffffea0002d96000 ffff880119b9f918 ffffffff81d6ce81 ffff88011a804480
ffff8800b6581500 ffff880119b9f948 ffffffff8146c7bd ffff88011a804480
ffffea0002d96000 ffff8800b6581500 fffffffffffffff4 ffff880119b9f970
Call Trace:
[<ffffffff81d6ce81>] dump_stack+0x65/0x84
[<ffffffff8146c7bd>] print_trailer+0x10d/0x1a0
[<ffffffff814704ff>] object_err+0x2f/0x40
[<ffffffff814754d1>] kasan_report_error+0x221/0x520
[<ffffffff8147590e>] __asan_report_load8_noabort+0x3e/0x40
[<ffffffff83888161>] klist_iter_exit+0x61/0x70
[<ffffffff82404389>] class_dev_iter_exit+0x9/0x10
[<ffffffff81d2e8ea>] disk_seqf_stop+0x3a/0x50
[<ffffffff8151f812>] seq_read+0x4b2/0x11a0
[<ffffffff815f8fdc>] proc_reg_read+0xbc/0x180
[<ffffffff814b24e4>] do_loop_readv_writev+0x134/0x210
[<ffffffff814b4c45>] do_readv_writev+0x565/0x660
[<ffffffff814b8a17>] vfs_readv+0x67/0xa0
[<ffffffff814b8de6>] do_preadv+0x126/0x170
[<ffffffff814b92ec>] SyS_preadv+0xc/0x10
This problem can occur in the following situation:
open()
- pread()
- .seq_start()
- iter = kmalloc() // succeeds
- seqf->private = iter
- .seq_stop()
- kfree(seqf->private)
- pread()
- .seq_start()
- iter = kmalloc() // fails
- .seq_stop()
- class_dev_iter_exit(seqf->private) // boom! old pointer
As the comment in disk_seqf_stop() says, stop is called even if start
failed, so we need to reinitialise the private pointer to NULL when seq
iteration stops.
An alternative would be to set the private pointer to NULL when the
kmalloc() in disk_seqf_start() fails.
Applied for 4.8, thanks.
--
Jens Axboe
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