[SRU][CVE-2020-29372][X][PATCH 1/1] mm: check that mm is still valid in madvise()

[William Breathitt Gray]

From: Linus Torvalds <torvalds at linux-foundation.org>

IORING_OP_MADVISE can end up basically doing mprotect() on the VM of
another process, which means that it can race with our crazy core dump
handling which accesses the VM state without holding the mmap_sem
(because it incorrectly thinks that it is the final user).

This is clearly a core dumping problem, but we've never fixed it the
right way, and instead have the notion of "check that the mm is still
ok" using mmget_still_valid() after getting the mmap_sem for writing in
any situation where we're not the original VM thread.

See commit 04f5866e41fb ("coredump: fix race condition between
mmget_not_zero()/get_task_mm() and core dumping") for more background on
this whole mmget_still_valid() thing.  You might want to have a barf bag
handy when you do.

We're discussing just fixing this properly in the only remaining core
dumping routines.  But even if we do that, let's make do_madvise() do
the right thing, and then when we fix core dumping, we can remove all
these mmget_still_valid() checks.

Reported-and-tested-by: Jann Horn <jannh at google.com>
Fixes: c1ca757bd6f4 ("io_uring: add IORING_OP_MADVISE")
Acked-by: Jens Axboe <axboe at kernel.dk>
Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>

CVE-2020-29372

(backported from commit bc0c4d1e176eeb614dc8734fc3ace34292771f11)
[ vilhelmgray: include linux/sched.h instead of linux/sched/mm.h ]
Signed-off-by: William Breathitt Gray <william.gray at canonical.com>
---
 mm/madvise.c | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/mm/madvise.c b/mm/madvise.c
index f548c66154ee..cf2b217a647a 100644
--- a/mm/madvise.c
+++ b/mm/madvise.c
@@ -20,6 +20,7 @@
 #include <linux/backing-dev.h>
 #include <linux/swap.h>
 #include <linux/swapops.h>
+#include <linux/sched.h>
 
 /*
  * Any behaviour which results in changes to the vma->vm_flags needs to
@@ -491,6 +492,23 @@ SYSCALL_DEFINE3(madvise, unsigned long, start, size_t, len_in, int, behavior)
 	write = madvise_need_mmap_write(behavior);
 	if (write)
 		down_write(&current->mm->mmap_sem);
+
+		/*
+		 * We may have stolen the mm from another process
+		 * that is undergoing core dumping.
+		 *
+		 * Right now that's io_ring, in the future it may
+		 * be remote process management and not "current"
+		 * at all.
+		 *
+		 * We need to fix core dumping to not do this,
+		 * but for now we have the mmget_still_valid()
+		 * model.
+		 */
+		if (!mmget_still_valid(current->mm)) {
+			up_write(&current->mm->mmap_sem);
+			return -EINTR;
+		}
 	else
 		down_read(&current->mm->mmap_sem);
 
-- 
2.27.0