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
