linux-kernel - Re: [syzbot] [mm?] general protection fault in find_mergeable_anon

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Message-ID: <c946c9d2-aff3-4492-99d1-d50e6e2659f6@lucifer.local>
Date: Mon, 9 Dec 2024 17:09:13 +0000
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
To: "Liam R. Howlett" <Liam.Howlett@...cle.com>, mhiramat@...nel.org,
        oleg@...hat.com, peterz@...radead.org, Jann Horn <jannh@...gle.com>,
        syzbot <syzbot+2d788f4f7cb660dac4b7@...kaller.appspotmail.com>,
        akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, syzkaller-bugs@...glegroups.com, vbabka@...e.cz
Subject: Re: [syzbot] [mm?] general protection fault in
 find_mergeable_anon_vma

On Mon, Dec 09, 2024 at 11:12:52AM -0500, Liam R. Howlett wrote:
> +Cc maintainers listed of kernel/events/uprobe.c
>
> TL;DR:
> dup_mmap() fails, but uprobe thinks it's fine and keeps trying to use an
> incomplete mm_struct.
>
> We're looking for a way to signal to uprobe to abort, cleanly.
>
> Looking at kernel/fork.c, dup_mmap():
>
> fail_uprobe_end:
>         uprobe_end_dup_mmap();
>         return retval;
>
> So uprobe is aware it could fail, but releases the semaphore and then
> doesn't check if the mm struct is okay to use.
>
> What should happen in the failed mm_struct case?
>
> Thanks,
> Liam
>

(As discussed on IRC) how about moving up the dup_mmap_sem lock one level, we
can put the mm before the rmap lookup in build_map_info() is able to find it,
which should avoid the whole issue?

Untested patch attached.

----8<----
>From 629b04fe8cfdf6b4fad0ff91a316d4643ccd737d Mon Sep 17 00:00:00 2001
From: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Date: Mon, 9 Dec 2024 16:58:14 +0000
Subject: [PATCH] tmp

---
 kernel/fork.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/kernel/fork.c b/kernel/fork.c
index 1450b461d196..4d62e776c413 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -639,7 +639,6 @@ static __latent_entropy int dup_mmap(struct mm_struct *mm,
 	LIST_HEAD(uf);
 	VMA_ITERATOR(vmi, mm, 0);

-	uprobe_start_dup_mmap();
 	if (mmap_write_lock_killable(oldmm)) {
 		retval = -EINTR;
 		goto fail_uprobe_end;
@@ -783,7 +782,6 @@ static __latent_entropy int dup_mmap(struct mm_struct *mm,
 	else
 		dup_userfaultfd_fail(&uf);
 fail_uprobe_end:
-	uprobe_end_dup_mmap();
 	return retval;

 fail_nomem_anon_vma_fork:
@@ -1692,9 +1690,11 @@ static struct mm_struct *dup_mm(struct task_struct *tsk,
 	if (!mm_init(mm, tsk, mm->user_ns))
 		goto fail_nomem;

+	uprobe_start_dup_mmap();
 	err = dup_mmap(mm, oldmm);
 	if (err)
 		goto free_pt;
+	uprobe_end_dup_mmap();

 	mm->hiwater_rss = get_mm_rss(mm);
 	mm->hiwater_vm = mm->total_vm;
@@ -1709,6 +1709,7 @@ static struct mm_struct *dup_mm(struct task_struct *tsk,
 	mm->binfmt = NULL;
 	mm_init_owner(mm, NULL);
 	mmput(mm);
+	uprobe_end_dup_mmap();

 fail_nomem:
 	return NULL;
--
2.47.1