> On Mar 31, 2021, at 11:48 PM, Song Liu <songliubraving@xxxxxx> wrote:
>
>
>
>> On Mar 31, 2021, at 5:07 PM, Dave Marchevsky <davemarchevsky@xxxxxx> wrote:
>>
>> On x86 the struct pt_regs * grabbed by task_pt_regs() points to an
>> offset of task->stack. The pt_regs are later dereferenced in
>> __bpf_get_stack (e.g. by user_mode() check). This can cause a fault if
>> the task in question exits while bpf_get_task_stack is executing, as
>> warned by task_stack_page's comment:
>>
>> * When accessing the stack of a non-current task that might exit, use
>> * try_get_task_stack() instead. task_stack_page will return a pointer
>> * that could get freed out from under you.
>>
>> Taking the comment's advice and using try_get_task_stack() and
>> put_task_stack() to hold task->stack refcount, or bail early if it's
>> already 0. Incrementing stack_refcount will ensure the task's stack
>> sticks around while we're using its data.
>>
>> I noticed this bug while testing a bpf task iter similar to
>> bpf_iter_task_stack in selftests, except mine grabbed user stack, and
>> getting intermittent crashes, which resulted in dumps like:
>>
>> BUG: unable to handle page fault for address: 0000000000003fe0
>> \#PF: supervisor read access in kernel mode
>> \#PF: error_code(0x0000) - not-present page
>> RIP: 0010:__bpf_get_stack+0xd0/0x230
>> <snip...>
>> Call Trace:
>> bpf_prog_0a2be35c092cb190_get_task_stacks+0x5d/0x3ec
>> bpf_iter_run_prog+0x24/0x81
>> __task_seq_show+0x58/0x80
>> bpf_seq_read+0xf7/0x3d0
>> vfs_read+0x91/0x140
>> ksys_read+0x59/0xd0
>> do_syscall_64+0x48/0x120
>> entry_SYSCALL_64_after_hwframe+0x44/0xa9
>>
>> Fixes: fa28dcb82a38 ("bpf: Introduce helper bpf_get_task_stack()")
>> Signed-off-by: Dave Marchevsky <davemarchevsky@xxxxxx>
>
> Thanks for the fix!
>
> Acked-by: Song Liu <songliubraving@xxxxxx>
>
> Could you please extend bpf_iter_task_stack to also grab user stack?
I think we can extend bpf_iter_task_stack in a follow up patch. It is
not necessary to bundle these two patches in the same set.
Thanks,
Song
