arm64: ptrace: fix partial SETREGSET for NT_ARM_POE

Currently poe_set() doesn't initialize the temporary 'ctrl' variable,
and a SETREGSET call with a length of zero will leave this
uninitialized. Consequently an arbitrary value will be written back to
target->thread.por_el0, potentially leaking up to 64 bits of memory from
the kernel stack. The read is limited to a specific slot on the stack,
and the issue does not provide a write mechanism.

Fix this by initializing the temporary value before copying the regset
from userspace, as for other regsets (e.g. NT_PRSTATUS, NT_PRFPREG,
NT_ARM_SYSTEM_CALL). In the case of a zero-length write, the existing
contents of POR_EL1 will be retained.

Before this patch:

| # ./poe-test
| Attempting to write NT_ARM_POE::por_el0 = 0x900d900d900d900d
| SETREGSET(nt=0x40f, len=8) wrote 8 bytes
|
| Attempting to read NT_ARM_POE::por_el0
| GETREGSET(nt=0x40f, len=8) read 8 bytes
| Read NT_ARM_POE::por_el0 = 0x900d900d900d900d
|
| Attempting to write NT_ARM_POE (zero length)
| SETREGSET(nt=0x40f, len=0) wrote 0 bytes
|
| Attempting to read NT_ARM_POE::por_el0
| GETREGSET(nt=0x40f, len=8) read 8 bytes
| Read NT_ARM_POE::por_el0 = 0xffff8000839c3d50

After this patch:

| # ./poe-test
| Attempting to write NT_ARM_POE::por_el0 = 0x900d900d900d900d
| SETREGSET(nt=0x40f, len=8) wrote 8 bytes
|
| Attempting to read NT_ARM_POE::por_el0
| GETREGSET(nt=0x40f, len=8) read 8 bytes
| Read NT_ARM_POE::por_el0 = 0x900d900d900d900d
|
| Attempting to write NT_ARM_POE (zero length)
| SETREGSET(nt=0x40f, len=0) wrote 0 bytes
|
| Attempting to read NT_ARM_POE::por_el0
| GETREGSET(nt=0x40f, len=8) read 8 bytes
| Read NT_ARM_POE::por_el0 = 0x900d900d900d900d

Fixes: 1751981 ("arm64/ptrace: add support for FEAT_POE")
Cc: <[email protected]> # 6.12.x
Signed-off-by: Mark Rutland <[email protected]>
Cc: Joey Gouly <[email protected]>
Cc: Will Deacon <[email protected]>
Reviewed-by: Mark Brown <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Catalin Marinas <[email protected]>

Author: mrutland-arm

arch/arm64/kernel/ptrace.c

@@ -1478,6 +1478,8 @@ static int poe_set(struct task_struct *target, const struct
 	if (!system_supports_poe())
 		return -EINVAL;
 
+	ctrl = target->thread.por_el0;
+
 	ret = user_regset_copyin(&pos, &count, &kbuf, &ubuf, &ctrl, 0, -1);
 	if (ret)
 		return ret;