Re: [PATCH] ui/cursor: incomplete check for integer overflow in cursor_alloc

["Daniel P. Berrangé" <berrange@redhat.com>]

On Mon, May 22, 2023 at 08:55:02PM +0200, Philippe Mathieu-Daudé wrote:
> On 9/5/23 09:13, Marc-André Lureau wrote:
> > Hi
> > 
> > On Mon, May 8, 2023 at 6:21 PM Mauro Matteo Cascella
> > <mcascell@redhat.com <mailto:mcascell@redhat.com>> wrote:
> > 
> >     The cursor_alloc function still accepts a signed integer for both
> >     the cursor
> >     width and height. A specially crafted negative width/height could
> >     make datasize
> >     wrap around and cause the next allocation to be 0, potentially
> >     leading to a
> >     heap buffer overflow. Modify QEMUCursor struct and cursor_alloc
> >     prototype to
> >     accept unsigned ints.
> > 
> >     Fixes: CVE-2023-1601
> >     Fixes: fa892e9a ("ui/cursor: fix integer overflow in cursor_alloc
> >     (CVE-2021-4206)")
> >     Signed-off-by: Mauro Matteo Cascella <mcascell@redhat.com
> >     <mailto:mcascell@redhat.com>>
> >     Reported-by: Jacek Halon <jacek.halon@gmail.com
> >     <mailto:jacek.halon@gmail.com>>
> > 
> > 
> > Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com
> > <mailto:marcandre.lureau@redhat.com>>
> > 
> > It looks like this is not exploitable, QXL code uses u16 types, and
> 
> 0xffff * 0xffff * 4 still overflows on 32-bit host, right?

cursor_alloc() will reject 0xffff:

    if (width > 512 || height > 512) {
        return NULL;
    }



> 
> > VMWare VGA checks for values > 256. Other paths use fixed size.
> > 
> >     ---
> >       include/ui/console.h | 4 ++--
> >       ui/cursor.c          | 2 +-
> >       2 files changed, 3 insertions(+), 3 deletions(-)
> > 
> >     diff --git a/include/ui/console.h b/include/ui/console.h
> >     index 2a8fab091f..92a4d90a1b 100644
> >     --- a/include/ui/console.h
> >     +++ b/include/ui/console.h
> >     @@ -144,13 +144,13 @@ typedef struct QemuUIInfo {
> > 
> >       /* cursor data format is 32bit RGBA */
> >       typedef struct QEMUCursor {
> >     -    int                 width, height;
> >     +    uint32_t            width, height;
> >           int                 hot_x, hot_y;
> >           int                 refcount;
> >           uint32_t            data[];
> >       } QEMUCursor;
> > 
> >     -QEMUCursor *cursor_alloc(int width, int height);
> >     +QEMUCursor *cursor_alloc(uint32_t width, uint32_t height);
> >       QEMUCursor *cursor_ref(QEMUCursor *c);
> >       void cursor_unref(QEMUCursor *c);
> >       QEMUCursor *cursor_builtin_hidden(void);
> >     diff --git a/ui/cursor.c b/ui/cursor.c
> >     index 6fe67990e2..b5fcb64839 100644
> >     --- a/ui/cursor.c
> >     +++ b/ui/cursor.c
> >     @@ -90,7 +90,7 @@ QEMUCursor *cursor_builtin_left_ptr(void)
> >           return cursor_parse_xpm(cursor_left_ptr_xpm);
> >       }
> > 
> >     -QEMUCursor *cursor_alloc(int width, int height)
> >     +QEMUCursor *cursor_alloc(uint32_t width, uint32_t height)
> >       {
> >           QEMUCursor *c;
> 
> Can't we check width/height > 0 && <= SOME_LIMIT_THAT_MAKES_SENSE?
> 
> Maybe a 16K * 16K cursor is future proof and safe enough.
> 
> >           size_t datasize = width * height * sizeof(uint32_t);
> >     --     2.40.1
> > 
> > 
> > 
> > 
> > -- 
> > Marc-André Lureau
> 
> 

With regards,
Daniel
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