Re: [PATCH] hw/9pfs: fix O_TRUNC bypass on read-only export

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

On Wed, Jul 15, 2026 at 06:10:00PM +0200, Christian Schoenebeck wrote:
> Guest 9p client opening a file with O_TRUNC on a read-only 9p file
> system using 9p2000.u protocol version, allowed to bypass 9p
> server's read-only check, eventually causing file(s) being
> truncated to empty file(s) on host's read-only export.
> 
> Root cause is that 9p server's read-only check is using Linux open
> flags like O_WRONLY, O_RDWR, O_TRUNC, but checking them against
> the 9p Topen request's "mode" parameter, which has a different
> encoding (Otrunc = 0x10 vs. O_TRUNC = 0x200).
> 
> Fix this by checking against the "flags" variable instead of the
> protocol's "mode" option. Because the "flags" variable is already
> converted to Linux encoding by omode_to_uflags() for 9p2000.u and
> by get_dotl_openflags() for 9p2000.L protocol version.
> 
> Only 9p2000.u was affected by this bypass, 9p2000.L uses the Linux
> format on protocol level already.
> 
> Fixes: 2c74c2cb4b ("hw/9pfs: Read-only support for 9p export")
> Resolves: https://gitlab.com/qemu-project/qemu/-/issues/4000
> Signed-off-by: Christian Schoenebeck <qemu_oss@crudebyte.com>
> ---
>  hw/9pfs/9p.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)

Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>

Once a CVE is assigned to this in the gitlab issue, just include
that CVE ID in the commit message when doing a pull request.

With regards,
Daniel
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