We want to remove VM_DENYWRITE only currently only used when mapping the executable during exec. During exec, we already deny_write_access() the executable, however, after exec completes the VMAs mapped with VM_DENYWRITE effectively keeps write access denied via deny_write_access(). Let's deny write access when setting the MM exe_file. With this change, we can remove VM_DENYWRITE for mapping executables. This represents a minor user space visible change: sys_prctl(PR_SET_MM_EXE_FILE) can now fail if the file is already opened writable. Also, after sys_prctl(PR_SET_MM_EXE_FILE), the file cannot be opened writable. Note that we can already fail with -EACCES if the file doesn't have execute permissions. Signed-off-by: David Hildenbrand <david@xxxxxxxxxx> --- kernel/fork.c | 39 ++++++++++++++++++++++++++++++++++----- 1 file changed, 34 insertions(+), 5 deletions(-) diff --git a/kernel/fork.c b/kernel/fork.c index 6bd2e52bcdfb..5d904878f19b 100644 --- a/kernel/fork.c +++ b/kernel/fork.c @@ -476,6 +476,7 @@ static __latent_entropy int dup_mmap(struct mm_struct *mm, { struct vm_area_struct *mpnt, *tmp, *prev, **pprev; struct rb_node **rb_link, *rb_parent; + struct file *exe_file; int retval; unsigned long charge; LIST_HEAD(uf); @@ -493,7 +494,10 @@ static __latent_entropy int dup_mmap(struct mm_struct *mm, mmap_write_lock_nested(mm, SINGLE_DEPTH_NESTING); /* No ordering required: file already has been exposed. */ - RCU_INIT_POINTER(mm->exe_file, get_mm_exe_file(oldmm)); + exe_file = get_mm_exe_file(oldmm); + RCU_INIT_POINTER(mm->exe_file, exe_file); + if (exe_file) + deny_write_access(exe_file); mm->total_vm = oldmm->total_vm; mm->data_vm = oldmm->data_vm; @@ -638,8 +642,13 @@ static inline void mm_free_pgd(struct mm_struct *mm) #else static int dup_mmap(struct mm_struct *mm, struct mm_struct *oldmm) { + struct file *exe_file; + mmap_write_lock(oldmm); - RCU_INIT_POINTER(mm->exe_file, get_mm_exe_file(oldmm)); + exe_file = get_mm_exe_file(oldmm); + RCU_INIT_POINTER(mm->exe_file, exe_file); + if (exe_file) + deny_write_access(exe_file); mmap_write_unlock(oldmm); return 0; } @@ -1163,11 +1172,19 @@ void set_mm_exe_file(struct mm_struct *mm, struct file *new_exe_file) */ old_exe_file = rcu_dereference_raw(mm->exe_file); - if (new_exe_file) + if (new_exe_file) { get_file(new_exe_file); + /* + * exec code is required to deny_write_access() successfully, + * so this cannot fail + */ + deny_write_access(new_exe_file); + } rcu_assign_pointer(mm->exe_file, new_exe_file); - if (old_exe_file) + if (old_exe_file) { + allow_write_access(old_exe_file); fput(old_exe_file); + } } int atomic_set_mm_exe_file(struct mm_struct *mm, struct file *new_exe_file) @@ -1194,10 +1211,22 @@ int atomic_set_mm_exe_file(struct mm_struct *mm, struct file *new_exe_file) } /* set the new file, lockless */ + ret = deny_write_access(new_exe_file); + if (ret) + return -EACCES; get_file(new_exe_file); + old_exe_file = xchg(&mm->exe_file, new_exe_file); - if (old_exe_file) + if (old_exe_file) { + /* + * Don't race with dup_mmap() getting the file and disallowing + * write access while someone might open the file writable. + */ + mmap_read_lock(mm); + allow_write_access(old_exe_file); fput(old_exe_file); + mmap_read_unlock(mm); + } return 0; } -- 2.31.1