Christophe Leroy wrote:
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.
Sure, I don't have an opinion on that, but I think this patch will still
be needed for -stable.
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 ?
I guess this won't be required if we end up with the change you are
proposing above?
- Naveen
