fs/proc/task_mmu: check p->vec_buf for NULL

commit 28aa299 upstream.

When the PAGEMAP_SCAN ioctl is 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 torvalds#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.

Link: https://lkml.kernel.org/r/20250922082206.6889-1-acsjakub@amazon.de
Fixes: 52526ca ("fs/proc/task_mmu: implement IOCTL to get and optionally clear info about PTEs")
Signed-off-by: Jakub Acs <acsjakub@amazon.de>
Reviewed-by: Muhammad Usama Anjum <usama.anjum@collabora.com>
Acked-by: David Hildenbrand <david@redhat.com>
Cc: Vlastimil Babka <vbabka@suse.cz>
Cc: Lorenzo Stoakes <lorenzo.stoakes@oracle.com>
Cc: Jinjiang Tu <tujinjiang@huawei.com>
Cc: Suren Baghdasaryan <surenb@google.com>
Cc: Penglei Jiang <superman.xpt@gmail.com>
Cc: Mark Brown <broonie@kernel.org>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Ryan Roberts <ryan.roberts@arm.com>
Cc: Andrei Vagin <avagin@gmail.com>
Cc: "Michał Mirosław" <mirq-linux@rere.qmqm.pl>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: Jakub Acs

fs/proc/task_mmu.c

@@ -2286,6 +2286,9 @@ 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