bpf: Guard against accessing NULL pt_regs in bpf_get_task_stack()

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@vger.kernel.org # v5.9+
Signed-off-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/d5ef83c361cc255494afd15ff1b4fb02a36e1dcf.1641468127.git.naveen.n.rao@linux.vnet.ibm.com

diff --git a/kernel/bpf/stackmap.c b/kernel/bpf/stackmap.c
index 49e5672..22c8ae9 100644 (file)
--- a/kernel/bpf/stackmap.c
+++ b/kernel/bpf/stackmap.c
@@ -472,13 +472,14 @@ BPF_CALL_4(bpf_get_task_stack, struct task_struct *, task, void *, buf,
           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;