[2/4] exec: Relocate S_ISREG() check
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Message ID 20200518055457.12302-3-keescook@chromium.org
State New
Headers show
Series
  • Relocate execve() sanity checks
Related show

Commit Message

Kees Cook May 18, 2020, 5:54 a.m. UTC
The execve(2)/uselib(2) syscalls have always rejected non-regular
files. Recently, it was noticed that a deadlock was introduced when trying
to execute pipes, as the S_ISREG() test was happening too late. This was
fixed in commit 73601ea5b7b1 ("fs/open.c: allow opening only regular files
during execve()"), but it was added after inode_permission() had already
run, which meant LSMs could see bogus attempts to execute non-regular
files. Move the test earlier.

Also include a comment with the redundant S_ISREG() checks at the end of
execve(2)/uselib(2) to note that they are present to avoid any mistakes.

Finally, instead of dereferencing the inode, use dcache for S_ISREG()
test.

My notes on the call path, and related arguments, checks, etc:

do_open_execat()
    struct open_flags open_exec_flags = {
        .open_flag = O_LARGEFILE | O_RDONLY | __FMODE_EXEC, ...
    do_filp_open(dfd, filename, open_flags)
        path_openat(nameidata, open_flags, flags)
            /* f_mode populated from open_flags in alloc_empty_file() */
            file = alloc_empty_file(open_flags, current_cred());
            do_open(nameidata, file, open_flags)
		/* new location of FMODE_EXEC vs S_ISREG() test */
                may_open(path, acc_mode, open_flag)
                    inode_permission(inode, MAY_OPEN | acc_mode)
                        security_inode_permission(inode, acc_mode)
                vfs_open(path, file)
                    do_dentry_open(file, path->dentry->d_inode, open)
                        /* old location of FMODE_EXEC vs S_ISREG() test */
                        security_file_open(f)
                        open()

Signed-off-by: Kees Cook <keescook@chromium.org>
---
 fs/exec.c  | 8 ++++++++
 fs/namei.c | 4 ++++
 fs/open.c  | 6 ------
 3 files changed, 12 insertions(+), 6 deletions(-)

Comments

Kees Cook June 4, 2020, 10:45 p.m. UTC | #1
On Mon, May 25, 2020 at 05:14:20PM +0800, kernel test robot wrote:
> Greeting,

(Whoops, I missed this in my inbox.)

> <<<test_start>>>
> tag=execveat02 stime=1590373229
> cmdline="execveat02"
> contacts=""
> analysis=exit
> <<<test_output>>>
> tst_test.c:1246: INFO: Timeout per run is 0h 05m 00s
> execveat02.c:64: PASS: execveat() fails as expected: EBADF (9)
> execveat02.c:64: PASS: execveat() fails as expected: EINVAL (22)
> execveat02.c:61: FAIL: execveat() fails unexpectedly, expected: ELOOP: EACCES (13)
> execveat02.c:64: PASS: execveat() fails as expected: ENOTDIR (20)

I will go check on this. Looking at the expected result (ELOOP) I think
this just means the test needs adjustment because it's trying to
double-check for a pathological case, but it seems their test setup
trips the (now earlier) IS_SREG() test. But I'll double-check and report
back!
Kees Cook June 5, 2020, 2:57 a.m. UTC | #2
On Mon, May 25, 2020 at 05:14:20PM +0800, kernel test robot wrote:
> execveat02.c:64: PASS: execveat() fails as expected: EBADF (9)
> execveat02.c:64: PASS: execveat() fails as expected: EINVAL (22)
> execveat02.c:61: FAIL: execveat() fails unexpectedly, expected: ELOOP: EACCES (13)
> execveat02.c:64: PASS: execveat() fails as expected: ENOTDIR (20)

tl;dr: I think this test is correct, and I think I see a way to improve
the offending patch series to do the right thing.


Okay, the LTP is checking for ELOOP on trying to exec a symlink:

...
 *    3) execveat() fails and returns ELOOP if the file identified by dirfd and
 *       pathname is a symbolic link and flag includes AT_SYMLINK_NOFOLLOW.
...
#define TESTDIR "testdir"
#define TEST_APP "execveat_errno"
...
#define TEST_SYMLINK "execveat_symlink"
...
#define TEST_ERL_SYMLINK TESTDIR"/"TEST_SYMLINK
...
        sprintf(app_sym_path, "%s/%s", cur_dir_path, TEST_ERL_SYMLINK);
...
        SAFE_SYMLINK(TEST_REL_APP, TEST_ERL_SYMLINK);

        fd = SAFE_OPEN(TEST_REL_APP, O_PATH);
...
static struct tcase {
        int *fd;
        char *pathname;
        int flag;
        int exp_err;
} tcases[] = {
...
        {&fd, app_sym_path, AT_SYMLINK_NOFOLLOW, ELOOP},
...
};
...
                TEST(execveat(*tc->fd, tc->pathname, argv, environ, tc->flag));

This is testing the exec _of_ a symlink under AT_SYMLINK_NOFOLLOW.

The execve(2) manpage says:

       ELOOP  Too many symbolic links were encountered in resolving
              pathname or  the  name  of  a script or ELF interpreter.

       ELOOP  The maximum recursion limit was reached during recursive
	      script interpretation (see "Interpreter scripts", above).
	      Before Linux 3.8, the error produced for this case was ENOEXEC.

Which actually doesn't mention this case. open(2) says:

       ELOOP  Too many symbolic links were encountered in resolving pathname.

       ELOOP  pathname was a symbolic link, and flags specified O_NOFOLLOW
	      but not O_PATH.

(but O_NOFOLLOW is limited to file creation. linkat(2) lists the AT_*
flags, and applied to openat, this seems to track: attempting to
execat where the final element is a symlink should fail with ELOOP,
though the manpage does warn that this makes it indistinguishable from
symlink loops -- the first item listed in the execve manpage for
ELOOP...)

Regardless, this does seem to be the "correct" result, as opening for
exec or opening just normally should really get the same error code.

The call path for execve looks like this:

    do_open_execat()
        struct open_flags open_exec_flags = {
            .open_flag = O_LARGEFILE | O_RDONLY | __FMODE_EXEC,
	    .acc_mode = MAY_READ | MAY_EXEC,
	    ...
        do_filp_open(dfd, filename, open_flags)
            path_openat(nameidata, open_flags, flags)
                file = alloc_empty_file(open_flags, current_cred());
		open_last_lookups(nd, file, open_flags)
		    step_into(nd, ...)
		        /* stop on symlink without LOOKUP_FOLLOW */
                do_open(nameidata, file, open_flags)
                    /* new location of FMODE_EXEC vs S_ISREG() test */
                    may_open(path, acc_mode, open_flag)
			/* test for S_IFLNK */
                        inode_permission(inode, MAY_OPEN | acc_mode)
                            security_inode_permission(inode, acc_mode)
                    vfs_open(path, file)
                        do_dentry_open(file, path->dentry->d_inode, open)
                            /* old location of FMODE_EXEC vs S_ISREG() test */
                            security_file_open(f)
                            open()

The step_into() is what kicks back out without LOOKUP_FOLLOW, so we're
left holding a symlink (S_IFMT inode). In do_open(), there is a set of
checks via may_open() which checks for S_IFMT and rejects it:

        switch (inode->i_mode & S_IFMT) {
        case S_IFLNK:
                return -ELOOP;

So that's the case LTP was testing for.

The patch in -next ("exec: relocate S_ISREG() check")[1], moves the regular
file requirement up before may_open(), for all the reasons mentioned in
the commit log (and the next patch[2]).

When I was originally trying to determine the best place for where the
checks should live, may_open() really did seem like the right place, but I
recognized that it was examining path characteristics (which was good) but
it didn't have the file, and that seemed to be an intentional separation.

What is needed in may_open() would be the "how was this file opened?"
piece of information: file->f_mode & FMODE_EXEC. However, in looking at
this again now, I wonder if it might be possible to use the MAY_EXEC
from the acc_mode? It seems the old check (in do_dentry_open() had no
access to the acc_mode, so it was forced to use the FMODE_EXEC signal
instead.

(I actually think this remains a bit of a design problem: path-based LSMs,
which see the opened file in security_file_open(), also don't have access
to acc_mode and must depend on the FMODE_* flags...)

I will respin the series to use acc_mode and move the tests into
may_open().

-Kees

[1] https://git.kernel.org/pub/scm/linux/kernel/git/kees/linux.git/commit/?h=vfs/fmode_exec/v1&id=36aaecef88b472ad5e0a408ffdb5b2e46f1478f6
[2] https://git.kernel.org/pub/scm/linux/kernel/git/kees/linux.git/commit/?h=vfs/fmode_exec/v1&id=a2f303b9f24ae2ecdc57f3db93e49b2b869893aa

Patch
diff mbox series

diff --git a/fs/exec.c b/fs/exec.c
index 30735ce1dc0e..f0c80a8b9ccd 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -139,6 +139,10 @@  SYSCALL_DEFINE1(uselib, const char __user *, library)
 	if (IS_ERR(file))
 		goto out;
 
+	/*
+	 * do_open() has already checked for this, but we can be extra
+	 * cautious and check again at the very end too.
+	 */
 	error = -EACCES;
 	if (!S_ISREG(file_inode(file)->i_mode))
 		goto exit;
@@ -860,6 +864,10 @@  static struct file *do_open_execat(int fd, struct filename *name, int flags)
 	if (IS_ERR(file))
 		goto out;
 
+	/*
+	 * do_open() has already checked for this, but we can be extra
+	 * cautious and check again at the very end too.
+	 */
 	err = -EACCES;
 	if (!S_ISREG(file_inode(file)->i_mode))
 		goto exit;
diff --git a/fs/namei.c b/fs/namei.c
index a320371899cf..b9408aacaaa4 100644
--- a/fs/namei.c
+++ b/fs/namei.c
@@ -3212,6 +3212,10 @@  static int do_open(struct nameidata *nd,
 	if ((nd->flags & LOOKUP_DIRECTORY) && !d_can_lookup(nd->path.dentry))
 		return -ENOTDIR;
 
+	/* Any file opened for execution has to be a regular file. */
+	if ((file->f_flags & FMODE_EXEC) && !d_is_reg(nd->path.dentry))
+		return -EACCES;
+
 	do_truncate = false;
 	acc_mode = op->acc_mode;
 	if (file->f_mode & FMODE_CREATED) {
diff --git a/fs/open.c b/fs/open.c
index 719b320ede52..bb16e4e3cd57 100644
--- a/fs/open.c
+++ b/fs/open.c
@@ -753,12 +753,6 @@  static int do_dentry_open(struct file *f,
 		return 0;
 	}
 
-	/* Any file opened for execve()/uselib() has to be a regular file. */
-	if (unlikely(f->f_flags & FMODE_EXEC && !S_ISREG(inode->i_mode))) {
-		error = -EACCES;
-		goto cleanup_file;
-	}
-
 	if (f->f_mode & FMODE_WRITE && !special_file(inode->i_mode)) {
 		error = get_write_access(inode);
 		if (unlikely(error))