arm64: ptrace: fix partial SETREGSET for NT_ARM_FPMR

arm64: ptrace: fix partial SETREGSET for NT_ARM_FPMR

commit f5d71291841aecfe5d8435da2dfa7f58ccd18bc8 upstream.

Currently fpmr_set() doesn't initialize the temporary 'fpmr' 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.uw.fpmr, 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 FPMR will be retained.

Before this patch:

| # ./fpmr-test
| Attempting to write NT_ARM_FPMR::fpmr = 0x900d900d900d900d
| SETREGSET(nt=0x40e, len=8) wrote 8 bytes
|
| Attempting to read NT_ARM_FPMR::fpmr
| GETREGSET(nt=0x40e, len=8) read 8 bytes
| Read NT_ARM_FPMR::fpmr = 0x900d900d900d900d
|
| Attempting to write NT_ARM_FPMR (zero length)
| SETREGSET(nt=0x40e, len=0) wrote 0 bytes
|
| Attempting to read NT_ARM_FPMR::fpmr
| GETREGSET(nt=0x40e, len=8) read 8 bytes
| Read NT_ARM_FPMR::fpmr = 0xffff800083963d50

After this patch:

| # ./fpmr-test
| Attempting to write NT_ARM_FPMR::fpmr = 0x900d900d900d900d
| SETREGSET(nt=0x40e, len=8) wrote 8 bytes
|
| Attempting to read NT_ARM_FPMR::fpmr
| GETREGSET(nt=0x40e, len=8) read 8 bytes
| Read NT_ARM_FPMR::fpmr = 0x900d900d900d900d
|
| Attempting to write NT_ARM_FPMR (zero length)
| SETREGSET(nt=0x40e, len=0) wrote 0 bytes
|
| Attempting to read NT_ARM_FPMR::fpmr
| GETREGSET(nt=0x40e, len=8) read 8 bytes
| Read NT_ARM_FPMR::fpmr = 0x900d900d900d900d

Fixes: 4035c22ef7d4 ("arm64/ptrace: Expose FPMR via ptrace")
Cc: <stable@vger.kernel.org> # 6.9.x
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Mark Brown <broonie@kernel.org>
Cc: Will Deacon <will@kernel.org>
Reviewed-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20241205121655.1824269-3-mark.rutland@arm.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>