Re: [PATCH v3] fs/proc/task_mmu: check p->vec_buf for NULL

[David Hildenbrand <david@redhat.com>]

On 22.09.25 10:22, Jakub Acs wrote:
> 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@amazon.de>

Acked-by: David Hildenbrand <david@redhat.com>

-- 
Cheers

David / dhildenb