crypto: ccp: Don't attempt to copy PDH cert to userspace if PSP command failed

When retrieving the PDH cert, don't attempt to copy the blobs to userspace
if the firmware command failed. If the failure was due to an invalid
length, i.e. the userspace buffer+length was too small, copying the number
of bytes _firmware_ requires will overflow the kernel-allocated buffer and
leak data to userspace.

BUG: KASAN: slab-out-of-bounds in instrument_copy_to_user ../include/linux/instrumented.h:129 [inline]
BUG: KASAN: slab-out-of-bounds in _inline_copy_to_user ../include/linux/uaccess.h:205 [inline]
BUG: KASAN: slab-out-of-bounds in _copy_to_user+0x66/0xa0 ../lib/usercopy.c:26
Read of size 2084 at addr ffff8885c4ab8aa0 by task syz.0.186/21033

CPU: 51 UID: 0 PID: 21033 Comm: syz.0.186 Tainted: G U O 7.0.0-smp-DEV #28 PREEMPTLAZY
Tainted: [U]=USER, [O]=OOT_MODULE
Hardware name: Google, Inc. Arcadia_IT_80/Arcadia_IT_80, BIOS 34.84.12-0 11/17/2025
Call Trace:
<TASK>
dump_stack_lvl+0xc5/0x110 ../lib/dump_stack.c:120
print_address_description ../mm/kasan/report.c:378 [inline]
print_report+0xbc/0x260 ../mm/kasan/report.c:482
kasan_report+0xa2/0xe0 ../mm/kasan/report.c:595
check_region_inline ../mm/kasan/generic.c:-1 [inline]
kasan_check_range+0x264/0x2c0 ../mm/kasan/generic.c:200
instrument_copy_to_user ../include/linux/instrumented.h:129 [inline]
_inline_copy_to_user ../include/linux/uaccess.h:205 [inline]
_copy_to_user+0x66/0xa0 ../lib/usercopy.c:26
copy_to_user ../include/linux/uaccess.h:236 [inline]
sev_ioctl_do_pdh_export+0x3d3/0x7c0 ../drivers/crypto/ccp/sev-dev.c:2347
sev_ioctl+0x2a2/0x490 ../drivers/crypto/ccp/sev-dev.c:2568
vfs_ioctl ../fs/ioctl.c:51 [inline]
__do_sys_ioctl ../fs/ioctl.c:597 [inline]
__se_sys_ioctl+0x11d/0x1b0 ../fs/ioctl.c:583
do_syscall_x64 ../arch/x86/entry/syscall_64.c:63 [inline]
do_syscall_64+0xe0/0x800 ../arch/x86/entry/syscall_64.c:94
entry_SYSCALL_64_after_hwframe+0x76/0x7e
</TASK>

WARN if the driver says the command succeeded, but the firmware error code
says otherwise, as __sev_do_cmd_locked() is expected to return -EIO on any
firwmware error.

Reported-by: Alexander Potapenko <glider@google.com>
Reported-by: Sebastian Alba Vives <sebasjosue84@gmail.com>
Fixes: 76a2b524a4b1 ("crypto: ccp: Implement SEV_PDH_CERT_EXPORT ioctl command")
Cc: stable@vger.kernel.org
Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>