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exec: Allow load_misc_binary to call prepare_binprm unconditionally

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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>
This commit is contained in:
Eric W. Biederman
2020-05-16 06:02:54 -05:00
parent 112b714759
commit a16b3357b2
3 changed files with 17 additions and 19 deletions
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19
fs/exec.c
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@@ -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);