linux-kernel - [PATCH v3] fs/proc/task_mmu: check p->vec

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Message-ID: <20250922082206.6889-1-acsjakub@amazon.de>
Date: Mon, 22 Sep 2025 08:22:05 +0000
From: Jakub Acs <acsjakub@...zon.de>
To: <linux-fsdevel@...r.kernel.org>
CC: <acsjakub@...zon.de>, Andrew Morton <akpm@...ux-foundation.org>, "David
 Hildenbrand" <david@...hat.com>, Vlastimil Babka <vbabka@...e.cz>, "Lorenzo
 Stoakes" <lorenzo.stoakes@...cle.com>, Jinjiang Tu <tujinjiang@...wei.com>,
	Suren Baghdasaryan <surenb@...gle.com>, Penglei Jiang
	<superman.xpt@...il.com>, Mark Brown <broonie@...nel.org>, Baolin Wang
	<baolin.wang@...ux.alibaba.com>, Ryan Roberts <ryan.roberts@....com>, "Andrei
 Vagin" <avagin@...il.com>, Michał Mirosław
	<mirq-linux@...e.qmqm.pl>, Stephen Rothwell <sfr@...b.auug.org.au>, "Muhammad
 Usama Anjum" <usama.anjum@...labora.com>, <linux-kernel@...r.kernel.org>,
	<stable@...r.kernel.org>
Subject: [PATCH v3] fs/proc/task_mmu: check p->vec_buf for NULL

When PAGEMAP_SCAN ioctl invoked with vec_len = 0 reaches
pagemap_scan_backout_range(), kernel panics with null-ptr-deref:

[   44.936808] Oops: general protection fault, probably for non-canonical address 0xdffffc0000000000: 0000 [#1] SMP DEBUG_PAGEALLOC KASAN NOPTI
[   44.937797] KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007]
[   44.938391] CPU: 1 UID: 0 PID: 2480 Comm: reproducer Not tainted 6.17.0-rc6 #22 PREEMPT(none)
[   44.939062] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.16.3-0-ga6ed6b701f0a-prebuilt.qemu.org 04/01/2014
[   44.939935] RIP: 0010:pagemap_scan_thp_entry.isra.0+0x741/0xa80

<snip registers, unreliable trace>

[   44.946828] Call Trace:
[   44.947030]  <TASK>
[   44.949219]  pagemap_scan_pmd_entry+0xec/0xfa0
[   44.952593]  walk_pmd_range.isra.0+0x302/0x910
[   44.954069]  walk_pud_range.isra.0+0x419/0x790
[   44.954427]  walk_p4d_range+0x41e/0x620
[   44.954743]  walk_pgd_range+0x31e/0x630
[   44.955057]  __walk_page_range+0x160/0x670
[   44.956883]  walk_page_range_mm+0x408/0x980
[   44.958677]  walk_page_range+0x66/0x90
[   44.958984]  do_pagemap_scan+0x28d/0x9c0
[   44.961833]  do_pagemap_cmd+0x59/0x80
[   44.962484]  __x64_sys_ioctl+0x18d/0x210
[   44.962804]  do_syscall_64+0x5b/0x290
[   44.963111]  entry_SYSCALL_64_after_hwframe+0x76/0x7e

vec_len = 0 in pagemap_scan_init_bounce_buffer() means no buffers are
allocated and p->vec_buf remains set to NULL.

This breaks an assumption made later in pagemap_scan_backout_range(),
that page_region is always allocated for p->vec_buf_index.

Fix it by explicitly checking p->vec_buf for NULL before dereferencing.

Other sites that might run into same deref-issue are already (directly
or transitively) protected by checking p->vec_buf.

Note:
From PAGEMAP_SCAN man page, it seems vec_len = 0 is valid when no output
is requested and it's only the side effects caller is interested in,
hence it passes check in pagemap_scan_get_args().

This issue was found by syzkaller.

Fixes: 52526ca7fdb9 ("fs/proc/task_mmu: implement IOCTL to get and optionally clear info about PTEs")
Signed-off-by: Jakub Acs <acsjakub@...zon.de>
Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: David Hildenbrand <david@...hat.com>
Cc: Vlastimil Babka <vbabka@...e.cz>
Cc: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>
Cc: Jinjiang Tu <tujinjiang@...wei.com>
Cc: Suren Baghdasaryan <surenb@...gle.com>
Cc: Penglei Jiang <superman.xpt@...il.com>
Cc: Mark Brown <broonie@...nel.org>
Cc: Baolin Wang <baolin.wang@...ux.alibaba.com>
Cc: Ryan Roberts <ryan.roberts@....com>
Cc: Andrei Vagin <avagin@...il.com>
Cc: "Michał Mirosław" <mirq-linux@...e.qmqm.pl>
Cc: Stephen Rothwell <sfr@...b.auug.org.au>
Cc: Muhammad Usama Anjum <usama.anjum@...labora.com>
Cc: linux-kernel@...r.kernel.org
Cc: linux-fsdevel@...r.kernel.org
Cc: stable@...r.kernel.org
---
v1 -> v2: check p->vec_buf instead of cur_buf
v2 -> v3: fix commit title

v1: https://lore.kernel.org/all/20250919142106.43527-1-acsjakub@amazon.de/ 
v2: https://lore.kernel.org/all/20250922081713.77303-1-acsjakub@amazon.de/

 fs/proc/task_mmu.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
index 29cca0e6d0ff..b26ae556b446 100644
--- a/fs/proc/task_mmu.c
+++ b/fs/proc/task_mmu.c
@@ -2417,6 +2417,9 @@ static void pagemap_scan_backout_range(struct pagemap_scan_private *p,
 {
 	struct page_region *cur_buf = &p->vec_buf[p->vec_buf_index];
 
+	if (!p->vec_buf)
+		return;
+
 	if (cur_buf->start != addr)
 		cur_buf->end = addr;
 	else
-- 
2.47.3




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