[PATCH 01/13] bpf: Guard against accessing NULL pt_regs in bpf_get_task_stack()

[Christophe Leroy]

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 at vger.kernel.org # v5.9+
> Signed-off-by: Naveen N. Rao <naveen.n.rao at linux.vnet.ibm.com>
> ---
>   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;