execve: open the executable file before doing anything else

No point in allocating a new mm, counting arguments and environment
variables etc if we're just going to return ENOENT.

This patch does expose the fact that 'do_filp_open()' that execve() uses
is still unnecessarily expensive in the failure case, because it
allocates the 'struct file *' early, even if the path lookup (which is
heavily optimized) fails.

So that remains an unnecessary cost in the "no such executable" case,
but it's a separate issue.  Regardless, I do not want to do _both_ a
filename_lookup() and a later do_filp_open() like the origin patch by
Josh Triplett did in [1].

Reported-by: Josh Triplett <josh@joshtriplett.org>
Cc: Kees Cook <keescook@chromium.org>
Cc: Mateusz Guzik <mjguzik@gmail.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Link: https://lore.kernel.org/lkml/5c7333ea4bec2fad1b47a8fa2db7c31e4ffc4f14.1663334978.git.josh@joshtriplett.org/
Link: https://lore.kernel.org/lkml/202209161637.9EDAF6B18@keescook/
Link: https://lore.kernel.org/lkml/CAHk-=wgznerM-xs+x+krDfE7eVBiy_HOam35rbsFMMOwvYuEKQ@mail.gmail.com/
Link: https://lore.kernel.org/lkml/CAHk-=whf9qLO8ipps4QhmS0BkM8mtWJhvnuDSdtw5gFjhzvKNA@mail.gmail.com/
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/fs/exec.c b/fs/exec.c
index 73e4045df271d148377340a40c77271ee36b2161..8cdd5b2dd09c2e8047d6bd360e14b060dd23fbf0 100644 (file)
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1508,12 +1508,24 @@ static void free_bprm(struct linux_binprm *bprm)
        kfree(bprm);
 }
 
-static struct linux_binprm *alloc_bprm(int fd, struct filename *filename)
+static struct linux_binprm *alloc_bprm(int fd, struct filename *filename, int flags)
 {
-       struct linux_binprm *bprm = kzalloc(sizeof(*bprm), GFP_KERNEL);
+       struct linux_binprm *bprm;
+       struct file *file;
        int retval = -ENOMEM;
-       if (!bprm)
-               goto out;
+
+       file = do_open_execat(fd, filename, flags);
+       if (IS_ERR(file))
+               return ERR_CAST(file);
+
+       bprm = kzalloc(sizeof(*bprm), GFP_KERNEL);
+       if (!bprm) {
+               allow_write_access(file);
+               fput(file);
+               return ERR_PTR(-ENOMEM);
+       }
+
+       bprm->file = file;
 
        if (fd == AT_FDCWD || filename->name[0] == '/') {
                bprm->filename = filename->name;
@@ -1526,18 +1538,28 @@ static struct linux_binprm *alloc_bprm(int fd, struct filename *filename)
                if (!bprm->fdpath)
                        goto out_free;
 
+               /*
+                * Record that a name derived from an O_CLOEXEC fd will be
+                * inaccessible after exec.  This allows the code in exec to
+                * choose to fail when the executable is not mmaped into the
+                * interpreter and an open file descriptor is not passed to
+                * the interpreter.  This makes for a better user experience
+                * than having the interpreter start and then immediately fail
+                * when it finds the executable is inaccessible.
+                */
+               if (get_close_on_exec(fd))
+                       bprm->interp_flags |= BINPRM_FLAGS_PATH_INACCESSIBLE;
+
                bprm->filename = bprm->fdpath;
        }
        bprm->interp = bprm->filename;
 
        retval = bprm_mm_init(bprm);
-       if (retval)
-               goto out_free;
-       return bprm;
+       if (!retval)
+               return bprm;
 
 out_free:
        free_bprm(bprm);
-out:
        return ERR_PTR(retval);
 }
 
@@ -1807,10 +1829,8 @@ static int exec_binprm(struct linux_binprm *bprm)
 /*
  * sys_execve() executes a new program.
  */
-static int bprm_execve(struct linux_binprm *bprm,
-                      int fd, struct filename *filename, int flags)
+static int bprm_execve(struct linux_binprm *bprm)
 {
-       struct file *file;
        int retval;
 
        retval = prepare_bprm_creds(bprm);
@@ -1826,26 +1846,8 @@ static int bprm_execve(struct linux_binprm *bprm,
        current->in_execve = 1;
        sched_mm_cid_before_execve(current);
 
-       file = do_open_execat(fd, filename, flags);
-       retval = PTR_ERR(file);
-       if (IS_ERR(file))
-               goto out_unmark;
-
        sched_exec();
 
-       bprm->file = file;
-       /*
-        * Record that a name derived from an O_CLOEXEC fd will be
-        * inaccessible after exec.  This allows the code in exec to
-        * choose to fail when the executable is not mmaped into the
-        * interpreter and an open file descriptor is not passed to
-        * the interpreter.  This makes for a better user experience
-        * than having the interpreter start and then immediately fail
-        * when it finds the executable is inaccessible.
-        */
-       if (bprm->fdpath && get_close_on_exec(fd))
-               bprm->interp_flags |= BINPRM_FLAGS_PATH_INACCESSIBLE;
-
        /* Set the unchanging part of bprm->cred */
        retval = security_bprm_creds_for_exec(bprm);
        if (retval)
@@ -1875,7 +1877,6 @@ out:
        if (bprm->point_of_no_return && !fatal_signal_pending(current))
                force_fatal_sig(SIGSEGV);
 
-out_unmark:
        sched_mm_cid_after_execve(current);
        current->fs->in_exec = 0;
        current->in_execve = 0;
@@ -1910,7 +1911,7 @@ static int do_execveat_common(int fd, struct filename *filename,
         * further execve() calls fail. */
        current->flags &= ~PF_NPROC_EXCEEDED;
 
-       bprm = alloc_bprm(fd, filename);
+       bprm = alloc_bprm(fd, filename, flags);
        if (IS_ERR(bprm)) {
                retval = PTR_ERR(bprm);
                goto out_ret;
@@ -1959,7 +1960,7 @@ static int do_execveat_common(int fd, struct filename *filename,
                bprm->argc = 1;
        }
 
-       retval = bprm_execve(bprm, fd, filename, flags);
+       retval = bprm_execve(bprm);
 out_free:
        free_bprm(bprm);
 
@@ -1984,7 +1985,7 @@ int kernel_execve(const char *kernel_filename,
        if (IS_ERR(filename))
                return PTR_ERR(filename);
 
-       bprm = alloc_bprm(fd, filename);
+       bprm = alloc_bprm(fd, filename, 0);
        if (IS_ERR(bprm)) {
                retval = PTR_ERR(bprm);
                goto out_ret;
@@ -2019,7 +2020,7 @@ int kernel_execve(const char *kernel_filename,
        if (retval < 0)
                goto out_free;
 
-       retval = bprm_execve(bprm, fd, filename, 0);
+       retval = bprm_execve(bprm);
 out_free:
        free_bprm(bprm);
 out_ret: