Re: [PATCH v3] qga: Change effective user/group ID in guest-ssh-* commands

On Mon, 10 Aug 2026 at 15:07, Kostiantyn Kostiuk <[email protected]> wrote:
>
> Before this commit, when qmp_guest_ssh_add_authorized_keys adds an
> SSH key for an existing local user, the agent (running as root) decides
> whether to create the user's .ssh directory with a symlink-following
> directory test, and then writes and chowns the authorized_keys file.
> A local unprivileged user who owns their home directory can pre-stage
> their .ssh directory (or the authorized_keys file) as a symbolic link
> so that, when the host or operator triggers a key add for that user,
> the root agent follows the link and transfers ownership of an arbitrary
> root-owned file or directory to the unprivileged user, who can then rewrite
> it to obtain root.
>
> Fixes: CVE-2026-12080
> Fixes: https://gitlab.com/qemu-project/qemu/-/work_items/3929
>
> v1: https://patchew.org/QEMU/[email protected]/
> v2 -> v1:
> Change effective user/group ID instead of checking for symlinks and
> changing ownership of the file.
>
> v2: https://patchew.org/QEMU/[email protected]
> v3 -> v2:
> Deduplicate code.
> Fail daemon when can't rollback effective user/group ID.
>
> Reported-by: Valentino Paulon <[email protected]>
> Signed-off-by: Kostiantyn Kostiuk <[email protected]>
> ---
>  qga/commands-posix-ssh.c | 66 ++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 66 insertions(+)
>
> diff --git a/qga/commands-posix-ssh.c b/qga/commands-posix-ssh.c
> index 661972e34e..070cdffcd0 100644
> --- a/qga/commands-posix-ssh.c
> +++ b/qga/commands-posix-ssh.c
> @@ -8,11 +8,29 @@
>  #include <glib/gstdio.h>
>  #include <locale.h>
>  #include <pwd.h>
> +#include <grp.h>
>
>  #include "commands-common-ssh.h"
>  #include "qapi/error.h"
>  #include "qga-qapi-commands.h"
>
> +typedef struct EffectiveUserInfo {
> +    uid_t uid;
> +    gid_t gid;
> +} EffectiveUserInfo;
> +
> +typedef EffectiveUserInfo *PEffectiveUserInfo;
> +
> +static void rollback_effective_info(PEffectiveUserInfo info)
> +{
> +    if (info) {
> +        assert(seteuid(info->uid) == 0);
> +        assert(setegid(info->gid) == 0);

A brief comment here about why we're happy to assert() here would
be helpful.

> +    }
> +}
> +
> +G_DEFINE_AUTO_CLEANUP_FREE_FUNC(PEffectiveUserInfo, rollback_effective_info, NULL);

Don't we also need to g_free(info) in the cleanup function?

thanks
-- PMM