[PATCH 5.15 012/171] bpf: Guard against accessing NULL pt_regs in bpf_get_task_stack()

[Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>]

From: Naveen N. Rao <naveen.n.rao@xxxxxxxxxxxxxxxxxx>

commit b992f01e66150fc5e90be4a96f5eb8e634c8249e upstream.

task_pt_regs() can return NULL on powerpc for kernel threads. This is
then used in __bpf_get_stack() to check for user mode, resulting in a
kernel oops. Guard against this by checking return value of
task_pt_regs() before trying to obtain the call chain.

Fixes: fa28dcb82a38f8 ("bpf: Introduce helper bpf_get_task_stack()")
Cc: stable@xxxxxxxxxxxxxxx # v5.9+
Signed-off-by: Naveen N. Rao <naveen.n.rao@xxxxxxxxxxxxxxxxxx>
Acked-by: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
Signed-off-by: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
Link: https://lore.kernel.org/r/d5ef83c361cc255494afd15ff1b4fb02a36e1dcf.1641468127.git.naveen.n.rao@xxxxxxxxxxxxxxxxxx
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 kernel/bpf/stackmap.c |    5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

--- a/kernel/bpf/stackmap.c
+++ b/kernel/bpf/stackmap.c
@@ -525,13 +525,14 @@ BPF_CALL_4(bpf_get_task_stack, struct ta
 	   u32, size, u64, flags)
 {
 	struct pt_regs *regs;
-	long res;
+	long res = -EINVAL;
 
 	if (!try_get_task_stack(task))
 		return -EFAULT;
 
 	regs = task_pt_regs(task);
-	res = __bpf_get_stack(regs, task, NULL, buf, size, flags);
+	if (regs)
+		res = __bpf_get_stack(regs, task, NULL, buf, size, flags);
 	put_task_stack(task);
 
 	return res;