Re: [syzbot] general protection fault in vma_interval_tree_remove

[Liam Howlett <liam.howlett@oracle.com>]

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* Liam R. Howlett <Liam.Howlett@Oracle.com> [220516 13:59]:
> * Andrew Morton <akpm@linux-foundation.org> [220514 16:50]:
> > On Sat, 14 May 2022 13:18:26 -0700 syzbot <syzbot+ee1fdd8dcc770a3a169a@syzkaller.appspotmail.com> wrote:
> > 
> > > syzbot has found a reproducer for the following issue on:
> > > 
> > > HEAD commit:    1e1b28b936ae Add linux-next specific files for 20220513
> > > git tree:       linux-next
> > > console+strace: https://syzkaller.appspot.com/x/log.txt?x=11da21b9f00000
> > > kernel config:  https://syzkaller.appspot.com/x/.config?x=e4eb3c0c4b289571
> > > dashboard link: https://syzkaller.appspot.com/bug?extid=ee1fdd8dcc770a3a169a
> > > compiler:       gcc (Debian 10.2.1-6) 10.2.1 20210110, GNU ld (GNU Binutils for Debian) 2.35.2
> > > syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=142757f1f00000
> > > C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=17cf0966f00000
> > 
> > Thanks.
> > 
> > So it was there on April 28 and it's there now.  Liam, do you think
> > anything in the mapletree changes could have perturbed the interval
> > tree handling?
> 
> It is certainly possible, these two trees are intertwined so much.  One
> area that sticks out as a possibility is vma_expand().  I created a
> vma_expand() function to handle growing a vma and potentially removing
> the next vma.  I do some interval tree modifications in there.
> 
> I'll add it to my list of items to look at.

This was my bug.  I reused a pointer that wasn't reused in this function
until I altered the error path in this commit.

Please apply this patch to the maple tree series to fix "mm/mmap: use
advanced maple tree API for mmap_region()"

Thanks,
Liam

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From 0a381753d0584ca02532889a5e84c482ecbc1b9a Mon Sep 17 00:00:00 2001
From: "Liam R. Howlett" <Liam.Howlett@Oracle.com>
Date: Wed, 13 Apr 2022 23:07:16 -0700
Subject: [PATCH] mm/mmap: Fix advanced maple tree API for mmap_region()

Overwriting next pointer in mmap_region() breaks the error path in a
future patch.

Signed-off-by: Liam R. Howlett <Liam.Howlett@Oracle.com>
---
 mm/mmap.c | 12 +++++-------
 1 file changed, 5 insertions(+), 7 deletions(-)

diff --git a/mm/mmap.c b/mm/mmap.c
index f705e68931ce..9e0c657c265d 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -1765,7 +1765,7 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
 {
 	struct mm_struct *mm = current->mm;
 	struct vm_area_struct *vma = NULL;
-	struct vm_area_struct *prev, *next;
+	struct vm_area_struct *next, *prev, *merge;
 	pgoff_t pglen = len >> PAGE_SHIFT;
 	unsigned long charged = 0;
 	unsigned long end = addr + len;
@@ -1883,19 +1883,17 @@ unsigned long mmap_region(struct file *file, unsigned long addr,
 		 * as we may succeed this time.
 		 */
 		if (unlikely(vm_flags != vma->vm_flags && prev)) {
-			next = vma_merge(mm, prev, vma->vm_start, vma->vm_end, vma->vm_flags,
+			merge = vma_merge(mm, prev, vma->vm_start, vma->vm_end, vma->vm_flags,
 				NULL, vma->vm_file, vma->vm_pgoff, NULL, NULL_VM_UFFD_CTX, NULL);
-			if (next) {
+			if (merge) {
 				/* ->mmap() can change vma->vm_file and fput the original file. So
 				 * fput the vma->vm_file here or we would add an extra fput for file
 				 * and cause general protection fault ultimately.
 				 */
 				fput(vma->vm_file);
 				vm_area_free(vma);
-				vma = prev;
-				/* Update vm_flags and possible addr to pick up the change. We don't
-				 * warn here if addr changed as the vma is not linked by vma_link().
-				 */
+				vma = merge;
+				/* Update vm_flags to pick up the change. */
 				addr = vma->vm_start;
 				vm_flags = vma->vm_flags;
 				goto unmap_writable;
-- 
2.35.1