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Subject[PATCH 4/7] exec: Move bprm_mm_init into alloc_bprm

Currently it is necessary for the usermode helper code and the code that
launches init to use set_fs so that pages coming from the kernel look like
they are coming from userspace.

To allow that usage of set_fs to be removed cleanly the argument copying
from userspace needs to happen earlier. Move the allocation and
initialization of bprm->mm into alloc_bprm so that the bprm->mm is
available early to store the new user stack into. This is a prerequisite
for copying argv and envp into the new user stack early before ther rest of
exec.

To keep the things consistent the cleanup of bprm->mm is moved into
free_bprm. So that bprm->mm will be cleaned up whenever bprm->mm is
allocated and free_bprm are called.

Moving bprm_mm_init earlier is safe as it does not depend on any files,
current->in_execve, current->fs->in_exec, bprm->unsafe, or the if the file
table is shared. (AKA bprm_mm_init does not depend on any of the code that
happens between alloc_bprm and where it was previously called.)

This moves bprm->mm cleanup after current->fs->in_exec is set to 0. This
is safe because current->fs->in_exec is only used to preventy taking an
additional reference on the fs_struct.

This moves bprm->mm cleanup after current->in_execve is set to 0. This is
safe because current->in_execve is only used by the lsms (apparmor and
tomoyou) and always for LSM specific functions, never for anything to do
with the mm.

This adds bprm->mm cleanup into the successful return path. This is safe
because being on the successful return path implies that begin_new_exec
succeeded and set brpm->mm to NULL. As bprm->mm is NULL bprm cleanup I am
moving into free_bprm will do nothing.

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
---
fs/exec.c | 16 ++++++++--------
1 file changed, 8 insertions(+), 8 deletions(-)

diff --git a/fs/exec.c b/fs/exec.c
index 7e8af27dd199..afb168bf5e23 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1543,6 +1543,10 @@ static int prepare_bprm_creds(struct linux_binprm *bprm)

static void free_bprm(struct linux_binprm *bprm)
{
+ if (bprm->mm) {
+ acct_arg_size(bprm, 0);
+ mmput(bprm->mm);
+ }
free_arg_pages(bprm);
if (bprm->cred) {
mutex_unlock(&current->signal->cred_guard_mutex);
@@ -1582,6 +1586,10 @@ static struct linux_binprm *alloc_bprm(int fd, struct filename *filename)
bprm->filename = bprm->fdpath;
}
bprm->interp = bprm->filename;
+
+ retval = bprm_mm_init(bprm);
+ if (retval)
+ goto out_free;
return bprm;

out_free:
@@ -1911,10 +1919,6 @@ static int do_execveat_common(int fd, struct filename *filename,
close_on_exec(fd, rcu_dereference_raw(current->files->fdt)))
bprm->interp_flags |= BINPRM_FLAGS_PATH_INACCESSIBLE;

- retval = bprm_mm_init(bprm);
- if (retval)
- goto out_unmark;
-
retval = prepare_arg_pages(bprm, argv, envp);
if (retval < 0)
goto out;
@@ -1962,10 +1966,6 @@ static int do_execveat_common(int fd, struct filename *filename,
*/
if (bprm->point_of_no_return && !fatal_signal_pending(current))
force_sigsegv(SIGSEGV);
- if (bprm->mm) {
- acct_arg_size(bprm, 0);
- mmput(bprm->mm);
- }

out_unmark:
current->fs->in_exec = 0;
--
2.25.0
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