exec: Allow load_misc_binary to call prepare_binprm unconditionally

Add a flag preserve_creds that binfmt_misc can set to prevent
credentials from being updated.  This allows binfmt_misc to always
call prepare_binprm.  Allowing the credential computation logic to be
consolidated.

Not replacing the credentials with the interpreters credentials is
safe because because an open file descriptor to the executable is
passed to the interpreter.   As the interpreter does not need to
reopen the executable it is guaranteed to see the same file that
exec sees.

Ref: c407c033de84 ("[PATCH] binfmt_misc: improve calculation of interpreter's credentials")
Link: https://lkml.kernel.org/r/87imgszrwo.fsf_-_@x220.int.ebiederm.org
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

diff --git a/fs/binfmt_misc.c b/fs/binfmt_misc.c
index cdb4582..2648297 100644 (file)
--- a/fs/binfmt_misc.c
+++ b/fs/binfmt_misc.c
@@ -218,19 +218,10 @@ static int load_misc_binary(struct linux_binprm *bprm)
                goto error;
 
        bprm->file = interp_file;
-       if (fmt->flags & MISC_FMT_CREDENTIALS) {
-               loff_t pos = 0;
-
-               /*
-                * No need to call prepare_binprm(), it's already been
-                * done.  bprm->buf is stale, update from interp_file.
-                */
-               memset(bprm->buf, 0, BINPRM_BUF_SIZE);
-               retval = kernel_read(bprm->file, bprm->buf, BINPRM_BUF_SIZE,
-                               &pos);
-       } else
-               retval = prepare_binprm(bprm);
+       if (fmt->flags & MISC_FMT_CREDENTIALS)
+               bprm->preserve_creds = 1;
 
+       retval = prepare_binprm(bprm);
        if (retval < 0)
                goto error;
 

diff --git a/fs/exec.c b/fs/exec.c
index 8e3b93d..028e0e3 100644 (file)
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1631,15 +1631,20 @@ static void bprm_fill_uid(struct linux_binprm *bprm)
  */
 int prepare_binprm(struct linux_binprm *bprm)
 {
-       int retval;
        loff_t pos = 0;
 
-       /* Recompute parts of bprm->cred based on bprm->file */
-       bprm->active_secureexec = 0;
-       bprm_fill_uid(bprm);
-       retval = security_bprm_repopulate_creds(bprm);
-       if (retval)
-               return retval;
+       /* Can the interpreter get to the executable without races? */
+       if (!bprm->preserve_creds) {
+               int retval;
+
+               /* Recompute parts of bprm->cred based on bprm->file */
+               bprm->active_secureexec = 0;
+               bprm_fill_uid(bprm);
+               retval = security_bprm_repopulate_creds(bprm);
+               if (retval)
+                       return retval;
+       }
+       bprm->preserve_creds = 0;
 
        memset(bprm->buf, 0, BINPRM_BUF_SIZE);
        return kernel_read(bprm->file, bprm->buf, BINPRM_BUF_SIZE, &pos);

diff --git a/include/linux/binfmts.h b/include/linux/binfmts.h
index 8605ab4..dbb5614 100644 (file)
--- a/include/linux/binfmts.h
+++ b/include/linux/binfmts.h
@@ -26,6 +26,8 @@ struct linux_binprm {
        unsigned long p; /* current top of mem */
        unsigned long argmin; /* rlimit marker for copy_strings() */
        unsigned int
+               /* It is safe to use the creds of a script (see binfmt_misc) */
+               preserve_creds:1,
                /*
                 * True if most recent call to security_bprm_set_creds
                 * resulted in elevated privileges.