[xen stable-4.17] x86/HVM: properly reject "indirect" VRAM writes

[patchbot@xxxxxxx]

commit d9606f9622bcbdd1f6761896c4c348d1fe53a45e
Author:     Jan Beulich <jbeulich@xxxxxxxx>
AuthorDate: Tue Nov 12 14:06:53 2024 +0100
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Tue Nov 12 14:06:53 2024 +0100

    x86/HVM: properly reject "indirect" VRAM writes
    
    While ->count will only be different from 1 for "indirect" (data in
    guest memory) accesses, it being 1 does not exclude the request being an
    "indirect" one. Check both to be on the safe side, and bring the ->count
    part also in line with what ioreq_send_buffered() actually refuses to
    handle.
    
    This is part of XSA-463 / CVE-2024-45818
    
    Fixes: 3bbaaec09b1b ("x86/hvm: unify stdvga mmio intercept with standard 
mmio intercept")
    Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
    Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    (cherry picked from commit eb7cd0593d88c4b967a24bca8bd30591966676cd)
---
 xen/arch/x86/hvm/stdvga.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/xen/arch/x86/hvm/stdvga.c b/xen/arch/x86/hvm/stdvga.c
index bed25e3cff..f5a4b1cfe9 100644
--- a/xen/arch/x86/hvm/stdvga.c
+++ b/xen/arch/x86/hvm/stdvga.c
@@ -499,13 +499,13 @@ static bool cf_check stdvga_mem_accept(
 
     spin_lock(&s->lock);
 
-    if ( p->dir == IOREQ_WRITE && p->count > 1 )
+    if ( p->dir == IOREQ_WRITE && (p->data_is_ptr || p->count != 1) )
     {
         /*
          * We cannot return X86EMUL_UNHANDLEABLE on anything other then the
          * first cycle of an I/O. So, since we cannot guarantee to always be
          * able to send buffered writes, we have to reject any multi-cycle
-         * I/O.
+         * or "indirect" I/O.
          */
         goto reject;
     }
--
generated by git-patchbot for /home/xen/git/xen.git#stable-4.17