mm: use down_read_killable for locking mmap_sem in access_remote_vm

This function is used by ptrace and proc files like /proc/pid/cmdline and
/proc/pid/environ.

Access_remote_vm never returns error codes, all errors are ignored and
only size of successfully read data is returned.  So, if current task was
killed we'll simply return 0 (bytes read).

Mmap_sem could be locked for a long time or forever if something goes
wrong.  Using a killable lock permits cleanup of stuck tasks and
simplifies investigation.

Link: http://lkml.kernel.org/r/156007494202.3335.16782303099589302087.stgit@buzz
Signed-off-by: Konstantin Khlebnikov <[email protected]>
Reviewed-by: Michal Koutný <[email protected]>
Acked-by: Oleg Nesterov <[email protected]>
Acked-by: Michal Hocko <[email protected]>
Cc: Alexey Dobriyan <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Cc: Cyrill Gorcunov <[email protected]>
Cc: Kirill Tkhai <[email protected]>
Cc: Al Viro <[email protected]>
Cc: Roman Gushchin <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

2 files changed, 5 insertions, 2 deletions

diff --git a/mm/memory.c b/mm/memory.c
index ad4bf1a1a0ef..53bd59579861 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -4344,7 +4344,9 @@ int __access_remote_vm(struct task_struct *tsk, struct mm_struct *mm,
 	void *old_buf = buf;
 	int write = gup_flags & FOLL_WRITE;
 
-	down_read(&mm->mmap_sem);
+	if (down_read_killable(&mm->mmap_sem))
+		return 0;
+
 	/* ignore errors, just check how much was successfully transferred */
 	while (len) {
 		int bytes, ret, offset;
diff --git a/mm/nommu.c b/mm/nommu.c
index 07165ad2e548..eb3e2e558da1 100644
--- a/mm/nommu.c
+++ b/mm/nommu.c
@@ -1704,7 +1704,8 @@ int __access_remote_vm(struct task_struct *tsk, struct mm_struct *mm,
 	struct vm_area_struct *vma;
 	int write = gup_flags & FOLL_WRITE;
 
-	down_read(&mm->mmap_sem);
+	if (down_read_killable(&mm->mmap_sem))
+		return 0;
 
 	/* the access must start within one of the target process's mappings */
 	vma = find_vma(mm, addr);