[xen stable-4.18] x86/HVM: properly reject "indirect" VRAM writes

[patchbot@xxxxxxx]

commit b7f66ed124985563c73dadeec84189c48870cd1a
Author:     Jan Beulich <jbeulich@xxxxxxxx>
AuthorDate: Tue Sep 24 15:00:07 2024 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Tue Sep 24 15:00:07 2024 +0200

    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.
    
    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>
    master commit: eb7cd0593d88c4b967a24bca8bd30591966676cd
    master date: 2024-09-12 09:13:04 +0200
---
 xen/arch/x86/hvm/stdvga.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/xen/arch/x86/hvm/stdvga.c b/xen/arch/x86/hvm/stdvga.c
index 2586891863..6419211266 100644
--- a/xen/arch/x86/hvm/stdvga.c
+++ b/xen/arch/x86/hvm/stdvga.c
@@ -530,14 +530,14 @@ 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 and, since we are rejecting an I/O, we must invalidate the
-         * cache.
+         * or "indirect" I/O and, since we are rejecting an I/O, we must
+         * invalidate the cache.
          * Single-cycle write transactions are accepted even if the cache is
          * not active since we can assert, when in stdvga mode, that writes
          * to VRAM have no side effect and thus we can try to buffer them.
--
generated by git-patchbot for /home/xen/git/xen.git#stable-4.18