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SubjectRe: [PATCH] apparmor: initialized returned struct aa_perms
On Fri, Sep 15, 2017 at 09:55:46PM +0200, Arnd Bergmann wrote:
> gcc-4.4 points out suspicious code in compute_mnt_perms, where
> the aa_perms structure is only partially initialized before getting
> returned:
>
> security/apparmor/mount.c: In function 'compute_mnt_perms':
> security/apparmor/mount.c:227: error: 'perms.prompt' is used uninitialized in this function
> security/apparmor/mount.c:227: error: 'perms.hide' is used uninitialized in this function
> security/apparmor/mount.c:227: error: 'perms.cond' is used uninitialized in this function
> security/apparmor/mount.c:227: error: 'perms.complain' is used uninitialized in this function
> security/apparmor/mount.c:227: error: 'perms.stop' is used uninitialized in this function
> security/apparmor/mount.c:227: error: 'perms.deny' is used uninitialized in this function
>
> Returning or assigning partially initialized structures is a bit tricky,
> in particular it is explicitly allowed in c99 to assign a partially
> intialized structure to another, as long as only members are read that
> have been initialized earlier. Looking at what various compilers do here,
> the version that produced the warning copied unintialized stack data,
> while newer versions (and also clang) either set the other members to
> zero or don't update the parts of the return buffer that are not modified
> in the temporary structure, but they never warn about this.
>
> In case of apparmor, it seems better to be a little safer and always
> initialize the aa_perms structure. Most users already do that, this
> changes the remaining ones, including the one instance that I got the
> warning for.
>
> Fixes: fa488437d0f9 ("apparmor: add mount mediation")
> Signed-off-by: Arnd Bergmann <arnd@arndb.de>

Reviewed-by: Seth Arnold <seth.arnold@canonical.com>

Thanks

> ---
> security/apparmor/file.c | 8 +-------
> security/apparmor/lib.c | 13 +++++--------
> security/apparmor/mount.c | 13 ++++++-------
> 3 files changed, 12 insertions(+), 22 deletions(-)
>
> diff --git a/security/apparmor/file.c b/security/apparmor/file.c
> index db80221891c6..86d57e56fabe 100644
> --- a/security/apparmor/file.c
> +++ b/security/apparmor/file.c
> @@ -227,18 +227,12 @@ static u32 map_old_perms(u32 old)
> struct aa_perms aa_compute_fperms(struct aa_dfa *dfa, unsigned int state,
> struct path_cond *cond)
> {
> - struct aa_perms perms;
> -
> /* FIXME: change over to new dfa format
> * currently file perms are encoded in the dfa, new format
> * splits the permissions from the dfa. This mapping can be
> * done at profile load
> */
> - perms.deny = 0;
> - perms.kill = perms.stop = 0;
> - perms.complain = perms.cond = 0;
> - perms.hide = 0;
> - perms.prompt = 0;
> + struct aa_perms perms = { };
>
> if (uid_eq(current_fsuid(), cond->uid)) {
> perms.allow = map_old_perms(dfa_user_allow(dfa, state));
> diff --git a/security/apparmor/lib.c b/security/apparmor/lib.c
> index 8818621b5d95..6cbc06da964c 100644
> --- a/security/apparmor/lib.c
> +++ b/security/apparmor/lib.c
> @@ -318,14 +318,11 @@ static u32 map_other(u32 x)
> void aa_compute_perms(struct aa_dfa *dfa, unsigned int state,
> struct aa_perms *perms)
> {
> - perms->deny = 0;
> - perms->kill = perms->stop = 0;
> - perms->complain = perms->cond = 0;
> - perms->hide = 0;
> - perms->prompt = 0;
> - perms->allow = dfa_user_allow(dfa, state);
> - perms->audit = dfa_user_audit(dfa, state);
> - perms->quiet = dfa_user_quiet(dfa, state);
> + *perms = (struct aa_perms) {
> + .allow = dfa_user_allow(dfa, state),
> + .audit = dfa_user_audit(dfa, state),
> + .quiet = dfa_user_quiet(dfa, state),
> + };
>
> /* for v5 perm mapping in the policydb, the other set is used
> * to extend the general perm set
> diff --git a/security/apparmor/mount.c b/security/apparmor/mount.c
> index 82a64b58041d..ed9b4d0f9f7e 100644
> --- a/security/apparmor/mount.c
> +++ b/security/apparmor/mount.c
> @@ -216,13 +216,12 @@ static unsigned int match_mnt_flags(struct aa_dfa *dfa, unsigned int state,
> static struct aa_perms compute_mnt_perms(struct aa_dfa *dfa,
> unsigned int state)
> {
> - struct aa_perms perms;
> -
> - perms.kill = 0;
> - perms.allow = dfa_user_allow(dfa, state);
> - perms.audit = dfa_user_audit(dfa, state);
> - perms.quiet = dfa_user_quiet(dfa, state);
> - perms.xindex = dfa_user_xindex(dfa, state);
> + struct aa_perms perms = {
> + .allow = dfa_user_allow(dfa, state),
> + .audit = dfa_user_audit(dfa, state),
> + .quiet = dfa_user_quiet(dfa, state),
> + .xindex = dfa_user_xindex(dfa, state),
> + };
>
> return perms;
> }
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