Le 06/01/2022 à 12:45, Naveen N. Rao a écrit :
> 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.
I started looking at that some time ago, and I'm wondering whether it is
worth keeping that powerpc particularity.
We used to have a potentially different pt_regs depending on how we
entered kernel, especially on PPC32, but since the following commits it
is not the case anymore.
06d67d54741a ("powerpc: make process.c suitable for both 32-bit and 64-bit")
db297c3b07af ("powerpc/32: Don't save thread.regs on interrupt entry")
b5cfc9cd7b04 ("powerpc/32: Fix critical and debug interrupts on BOOKE")
We could therefore just do like other architectures, define
#define task_pt_regs(p) ((struct pt_regs *)(THREAD_SIZE +
task_stack_page(p)) - 1)
And then remove the regs field we have in thread_struct.
>
> Fixes: fa28dcb82a38f8 ("bpf: Introduce helper bpf_get_task_stack()")
> Cc: stable@xxxxxxxxxxxxxxx # v5.9+
> Signed-off-by: Naveen N. Rao <naveen.n.rao@xxxxxxxxxxxxxxxxxx>
> ---
> kernel/bpf/stackmap.c | 5 +++--
> 1 file changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/kernel/bpf/stackmap.c b/kernel/bpf/stackmap.c
> index 6e75bbee39f0b5..0dcaed4d3f4cec 100644
> --- a/kernel/bpf/stackmap.c
> +++ b/kernel/bpf/stackmap.c
> @@ -525,13 +525,14 @@ 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);
Should there be a comment explaining that on powerpc, 'regs' can be NULL
for a kernel thread ?
> put_task_stack(task);
>
> return res;
