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    Subject[PATCH 5.16 021/200] bpf: Guard against accessing NULL pt_regs in bpf_get_task_stack()
    Date
    From: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>

    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@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
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    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;

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