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Subject[patch 12/55] proc: make sure mem_open() doesnt pin the targets memory
3.0-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Oleg Nesterov <oleg@redhat.com>

commit 6d08f2c7139790c268820a2e590795cb8333181a upstream.

Once /proc/pid/mem is opened, the memory can't be released until
mem_release() even if its owner exits.

Change mem_open() to do atomic_inc(mm_count) + mmput(), this only
pins mm_struct. Change mem_rw() to do atomic_inc_not_zero(mm_count)
before access_remote_vm(), this verifies that this mm is still alive.

I am not sure what should mem_rw() return if atomic_inc_not_zero()
fails. With this patch it returns zero to match the "mm == NULL" case,
may be it should return -EINVAL like it did before e268337d.

Perhaps it makes sense to add the additional fatal_signal_pending()
check into the main loop, to ensure we do not hold this memory if
the target task was oom-killed.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
fs/proc/base.c | 14 +++++++++++++-
1 file changed, 13 insertions(+), 1 deletion(-)

--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -775,6 +775,13 @@ static int mem_open(struct inode* inode,
if (IS_ERR(mm))
return PTR_ERR(mm);

+ if (mm) {
+ /* ensure this mm_struct can't be freed */
+ atomic_inc(&mm->mm_count);
+ /* but do not pin its memory */
+ mmput(mm);
+ }
+
/* OK to pass negative loff_t, we can catch out-of-range */
file->f_mode |= FMODE_UNSIGNED_OFFSET;
file->private_data = mm;
@@ -798,6 +805,9 @@ static ssize_t mem_rw(struct file *file,
return -ENOMEM;

copied = 0;
+ if (!atomic_inc_not_zero(&mm->mm_users))
+ goto free;
+
while (count > 0) {
int this_len = min_t(int, count, PAGE_SIZE);

@@ -825,6 +835,8 @@ static ssize_t mem_rw(struct file *file,
}
*ppos = addr;

+ mmput(mm);
+free:
free_page((unsigned long) page);
return copied;
}
@@ -861,7 +873,7 @@ static int mem_release(struct inode *ino
{
struct mm_struct *mm = file->private_data;
if (mm)
- mmput(mm);
+ mmdrop(mm);
return 0;
}




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