[PATCH] exec: Make suid_dumpable apply to SUID/SGID binaries irrespective of invoking users

Waiman Long
2021-12-21T02:18:07Z

The begin_new_exec() function checks for SUID or SGID binaries by
comparing effective uid and gid against real uid and gid and using
the suid_dumpable sysctl parameter setting only if either one of them
differs.

In the special case that the uid and/or gid of the SUID/SGID binaries
matches the id's of the user invoking it, the suid_dumpable is not
used and SUID_DUMP_USER will be used instead. The documentation for the
suid_dumpable sysctl parameter does not include that exception and so
this will be an undocumented behavior.

Eliminate this undocumented behavior by adding a flag in the linux_binprm
structure to designate a SUID/SGID binary and use it for determining
if the suid_dumpable setting should be applied or not.

Signed-off-by: Waiman Long <longman@redhat.com>
---
fs/exec.c | 6 +++---
include/linux/binfmts.h | 5 ++++-
2 files changed, 7 insertions(+), 4 deletions(-)

diff --git a/fs/exec.c b/fs/exec.c
index 537d92c41105..60e02e678fb6 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1344,9 +1344,7 @@ int begin_new_exec(struct linux_binprm * bprm)
* is wrong, but userspace depends on it. This should be testing
* bprm->secureexec instead.
*/
- if (bprm->interp_flags & BINPRM_FLAGS_ENFORCE_NONDUMP ||
- !(uid_eq(current_euid(), current_uid()) &&
- gid_eq(current_egid(), current_gid())))
+ if (bprm->interp_flags & BINPRM_FLAGS_ENFORCE_NONDUMP || bprm->is_sugid)
set_dumpable(current->mm, suid_dumpable);
else
set_dumpable(current->mm, SUID_DUMP_USER);
@@ -1619,11 +1617,13 @@ static void bprm_fill_uid(struct linux_binprm *bprm, struct file *file)
if (mode & S_ISUID) {
bprm->per_clear |= PER_CLEAR_ON_SETID;
bprm->cred->euid = uid;
+ bprm->is_sugid = 1;
}

if ((mode & (S_ISGID | S_IXGRP)) == (S_ISGID | S_IXGRP)) {
bprm->per_clear |= PER_CLEAR_ON_SETID;
bprm->cred->egid = gid;
+ bprm->is_sugid = 1;
}
}

diff --git a/include/linux/binfmts.h b/include/linux/binfmts.h
index 049cf9421d83..6d9893c59085 100644
--- a/include/linux/binfmts.h
+++ b/include/linux/binfmts.h
@@ -41,7 +41,10 @@ struct linux_binprm {
* Set when errors can no longer be returned to the
* original userspace.
*/
- point_of_no_return:1;
+ point_of_no_return:1,
+
+ /* Is a SUID or SGID binary? */
+ is_sugid:1;
#ifdef __alpha__
unsigned int taso:1;
#endif
--
2.27.0