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

commit abe4a6d6f606113251868c2c4a06ba904bb41eed upstream.

When retrieving the PEK CSR, don't attempt to copy the blob 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 ffff898144612e20 by task syz.9.219/21405

  CPU: 14 UID: 0 PID: 21405 Comm: syz.9.219 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 12.62.0-0 11/19/2025
  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_pek_csr+0x31f/0x590 ../drivers/crypto/ccp/sev-dev.c:1872
   sev_ioctl+0x3a4/0x490 ../drivers/crypto/ccp/sev-dev.c:2562
   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: e799035609 ("crypto: ccp: Implement SEV_PEK_CSR 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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
[uli: backport to 4.19]
Signed-off-by: Ulrich Hecht <uli@kernel.org>

drivers/crypto/ccp/psp-dev.c

@@ -387,7 +387,10 @@ cmd:
 
 	ret = __sev_do_cmd_locked(SEV_CMD_PEK_CSR, data, &argp->error);
 
-	 /* If we query the CSR length, FW responded with expected data. */
+	/*
+	 * Firmware will returns the length of the CSR blob (either the minimum
+	 * required length or the actual length written), return it to the user.
+	 */
 	input.length = data->len;
 
 	if (copy_to_user((void __user *)argp->data, &input, sizeof(input))) {
@@ -395,6 +398,9 @@ cmd:
 		goto e_free_blob;
 	}
 
+	if (ret || WARN_ON_ONCE(argp->error))
+		goto e_free_blob;
+
 	if (blob) {
 		if (copy_to_user((void __user *)input.address, blob, input.length))
 			ret = -EFAULT;