[PATCH] hw/display/vga: fix panning_buf OOB after text/graphics switch

From: Marc-André Lureau <[email protected]>

The fields last_width and last_height serve two purposes: the text
renderer counts in characters, the graphics renderer in pixels.
panning_buf reallocation is guarded by geometry-change check, so the
unit mismatch can trick it into thinking nothing changed when the
resolution actually grew.

A guest can trigger this by switching graphics -> text -> graphics:

  1. Enter graphics mode with a small width (CR01=0x00, 8 pixels).
     The predicate fires and panning_buf is allocated for that width.

  2. Switch to text mode with a large width (CR01=0xFF, 256 chars).
     The text renderer stores 256 into last_width. The text path
     never touches panning_buf.

  3. Switch back to graphics with a width that happens to equal 256
     in pixels (CR01=0x1F, 32*8 = 256). The predicate sees
     256 == 256 and skips the realloc. With horizontal pel panning
     enabled, vga_draw_line4() then writes a full 256-pixel scanline
     into the buffer still sized for 8 pixels -- a 960-byte heap
     overflow on every scanline, every refresh.

Fix it by reallocating unconditionally panning_buf on
vga_draw_graphic().

Fixes: CVE-2026-17516
Fixes: 973a724eb006 ("vga: implement horizontal pel panning in graphics modes")
Resolves: https://gitlab.com/qemu-project/qemu/-/work_items/4085
Cc: Paolo Bonzini <[email protected]>
Signed-off-by: Warisjeet Singh <[email protected]>
[ Marc- André - drop realloc() resize condition & commit message ]
Signed-off-by: Marc-André Lureau <[email protected]>
---
 hw/display/vga.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/hw/display/vga.c b/hw/display/vga.c
index abe3f8e07758..da0c331486eb 100644
--- a/hw/display/vga.c
+++ b/hw/display/vga.c
@@ -1647,11 +1647,12 @@ static void vga_draw_graphic(VGACommonState *s, int full_update)
         s->last_line_offset = s->params.line_offset;
         s->last_depth = depth;
         s->last_byteswap = byteswap;
-        /* 16 extra pixels are needed for double-width planar modes.  */
-        s->panning_buf = g_realloc(s->panning_buf,
-                                   (disp_width + 16) * sizeof(uint32_t));
         full_update = 1;
     }
+
+    /* 16 extra pixels are needed for double-width planar modes. */
+    s->panning_buf = g_realloc(s->panning_buf,
+                               (disp_width + 16) * sizeof(uint32_t));
     if (surface_data(surface) != s->vram_ptr + (s->params.start_addr * 4)
         && !surface_is_allocated(surface)) {
         /* base address changed (page flip) -> shared display surfaces
-- 
2.55.0