[PATCH for-4.15] x86/ucode/amd: Fix OoB read in cpu_request_microcode()

[Andrew Cooper <andrew.cooper3@citrix.com>]

verify_patch_size() is a maximum size check, and doesn't have a minimum bound.

If the microcode container encodes a blob with a length less than 64 bytes,
the subsequent calls to microcode_fits()/compare_header() may read off the end
of the buffer.

Fixes: 4de936a38a ("x86/ucode/amd: Rework parsing logic in cpu_request_microcode()")
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
---
CC: Jan Beulich <JBeulich@suse.com>
CC: Roger Pau Monné <roger.pau@citrix.com>
CC: Wei Liu <wl@xen.org>
CC: Ian Jackson <iwj@xenproject.org>

In practice, processor_rev_id is the only field read, which is 2 bytes at
offset 24 into the header.  Not that this makes the bug any less bad.

For 4.15.  Only dom0 can load new microcode, hence no XSA, but the bug is bad
and the fix simple and obvious.
---
 xen/arch/x86/cpu/microcode/amd.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/xen/arch/x86/cpu/microcode/amd.c b/xen/arch/x86/cpu/microcode/amd.c
index c4ab395799..cf5947389f 100644
--- a/xen/arch/x86/cpu/microcode/amd.c
+++ b/xen/arch/x86/cpu/microcode/amd.c
@@ -349,6 +349,7 @@ static struct microcode_patch *cpu_request_microcode(const void *buf, size_t siz
             if ( size < sizeof(*mc) ||
                  (mc = buf)->type != UCODE_UCODE_TYPE ||
                  size - sizeof(*mc) < mc->len ||
+                 mc->len < sizeof(struct microcode_patch) ||
                  (!skip_ucode && !verify_patch_size(mc->len)) )
             {
                 printk(XENLOG_ERR "microcode: Bad microcode data\n");
-- 
2.11.0