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>
---
kernel/bpf/stackmap.c | 12 ++++++++++--
1 file changed, 10 insertions(+), 2 deletions(-)
diff --git a/kernel/bpf/stackmap.c b/kernel/bpf/stackmap.c
index be35bfb7fb13..6fbc2abe9c91 100644
--- a/kernel/bpf/stackmap.c
+++ b/kernel/bpf/stackmap.c
@@ -517,9 +517,17 @@ const struct bpf_func_proto bpf_get_stack_proto = {
BPF_CALL_4(bpf_get_task_stack, struct task_struct *, task, void *, buf,
u32, size, u64, flags)
{
- struct pt_regs *regs = task_pt_regs(task);
+ struct pt_regs *regs;
+ long res;
- return __bpf_get_stack(regs, task, NULL, buf, size, flags);
+ if (!try_get_task_stack(task))
+ return -EFAULT;
+
+ regs = task_pt_regs(task);
+ res = __bpf_get_stack(regs, task, NULL, buf, size, flags);
+ put_task_stack(task);
+
+ return res;
}
BTF_ID_LIST_SINGLE(bpf_get_task_stack_btf_ids, struct, task_struct)
--
2.30.2
