CVE-2026-63794

In the Linux kernel, the following vulnerability has been resolved:

KVM: SVM: Fix page overflow in sev_dbg_crypt() for ENCRYPT path

In sev_dbg_crypt(), the per-iteration transfer length is bounded by
the source page offset (PAGE_SIZE - s_off) but not by the destination
page offset (PAGE_SIZE - d_off). When d_off > s_off, the encrypt
path (__sev_dbg_encrypt_user) performs a read-modify-write using a
single-page intermediate buffer (dst_tpage):

1. __sev_dbg_decrypt() expands the size to round_up(len + (d_off & 15), 16)
before issuing the PSP command. If len + (d_off & 15) > PAGE_SIZE,
the PSP writes beyond the end of the 4096-byte dst_tpage allocation.

2. The subsequent memcpy()/copy_from_user() into
page_address(dst_tpage) + (d_off & 15) of 'len' bytes overflows
by up to 15 bytes under the same condition.

Trigger example: s_off = 0, d_off = 1, debug.len = PAGE_SIZE -
the PSP is instructed to write round_up(4097, 16) = 4112 bytes to
a 4096-byte buffer.

Fix by also bounding len by (PAGE_SIZE - d_off), the same check that
sev_send_update_data() already performs for its single-page guest
region.

==================================================================
BUG: KASAN: slab-use-after-free in sev_dbg_crypt+0x993/0xd10 [kvm_amd]
Write of size 4095 at addr ff110062293bb009 by task sev_dbg_test/228214

CPU: 96 UID: 0 PID: 228214 Comm: sev_dbg_test Tainted: G U W 7.0.0-smp--5ce9b0c48211-dbg #156 PREEMPTLAZY
Tainted: [U]=USER, [W]=WARN
Hardware name: Google Astoria/astoria, BIOS 0.20250817.1-0 08/25/2025
Call Trace:
<TASK>
dump_stack_lvl+0x54/0x70
print_report+0xbc/0x260
kasan_report+0xa2/0xd0
kasan_check_range+0x25f/0x2c0
__asan_memcpy+0x40/0x70
sev_dbg_crypt+0x993/0xd10 [kvm_amd]
sev_mem_enc_ioctl+0x33c/0x450 [kvm_amd]
kvm_vm_ioctl+0x65d/0x6d0 [kvm]
__se_sys_ioctl+0xb2/0x100
do_syscall_64+0xe8/0x870
entry_SYSCALL_64_after_hwframe+0x4b/0x53
</TASK>

The buggy address belongs to the physical page:
page: refcount:1 mapcount:0 mapping:0000000000000000 index:0x7fe72b6a0 pfn:0x62293bb
memcg:ff11000112827d82
flags: 0x1400000000000000(node=1|zone=1)
raw: 1400000000000000 0000000000000000 dead000000000122 0000000000000000
raw: 00000007fe72b6a0 0000000000000000 00000001ffffffff ff11000112827d82
page dumped because: kasan: bad access detected

Memory state around the buggy address:
ff110062293bbf00: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
ff110062293bbf80: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
>ff110062293bc000: fa fb fb fb fb fb fb fb fc fc fc fc fc fc fc fc
^
ff110062293bc080: fa fb fb fb fb fb fb fb fc fc fc fc fc fc fc fc
ff110062293bc100: fa fb fb fb fb fb fb fb fc fc fc fc fc fc fc fc
==================================================================
Disabling lock debugging due to kernel taint

[sean: add sample KASAN splat, Fixes, and stable@]