Re: CVE-2026-80697: erofs: ensure valid f_path for page cache sharing

[Gao Xiang]

Hi Greg,

On Fri, Aug 28, 2026 at 08:53:20AM +0200, Greg Kroah-Hartman wrote:
> From: Greg Kroah-Hartman <gregkh@xxxxxxxxxx>
> 
> Description
> ===========
> 
> In the Linux kernel, the following vulnerability has been resolved:
> 
> erofs: ensure valid f_path for page cache sharing
> 
> Previously, backing files for page cache sharing were set up with
> f_path left as NULL (only f_inode was valid).  It worked, but a recent
> mincore fix relies on f_path.mnt and crashes (found by "erofs/028" on
> 7.2-rc4):
> 
>  BUG: kernel NULL pointer dereference, address: 0000000000000018
>  #PF: supervisor read access in kernel mode
>  #PF: error_code(0x0000) - not-present page
>  PGD 0 P4D 0
>  Oops: Oops: 0000 [#1] SMP PTI
>  CPU: 3 UID: 0 PID: 675528 Comm: fincore Not tainted 7.2.0-rc4-00002-g[]-dirty #1 PREEMPT(lazy)
>  Hardware name: Red Hat KVM, BIOS 1.16.0-4.al8 04/01/2014
>  RIP: 0010:__do_sys_mincore+0xc0/0x2c0
>  ...
> 
> Specify valid paths using valid disconnected dentries together with
> erofs_ishare_mnt instead of leaving f_path empty, so they are more
> like real backing files in a pseudo filesystem and standard
> backing_file_open() can be used directly.
> 
> The Linux kernel CVE team has assigned CVE-2026-80697 to this issue.
> 
> 
> Affected and fixed versions
> ===========================
> 
> 	Issue introduced in 7.1.4 with commit 04ba248d02d9eaa3d9077b00a6134caa75fa3e90 and fixed in 7.1.8 with commit 3879657c4ffd81b9a42cf575fc6cb60201032587
> 	Issue introduced in 5.15.217 with commit 7368bec565bac3e536cd43579dbde1e715e6ba61
> 	Issue introduced in 6.1.184 with commit b2f3d94ea310bea9d36d53e9d9b3f45e86c1d893
> 	Issue introduced in 6.6.145 with commit 744b23aa430d52f5c8e4dbff7d71496d6643bed2
> 	Issue introduced in 6.12.96 with commit 8344bdf0629457e532797b42d9d2bbf2a2900bbf
> 	Issue introduced in 6.18.39 with commit 5c942ad7df75925ee166e7f0fb36892d8dde376b

I don't think that is correct, although commit e187bc02f8fa
("mm: do file ownership checks with the proper mount idmap") directly
causes this issue, the entire page cache sharing feature was introduced
in Linux 7.0.

There is not any codebase below Linux 7.0 so it's possible to fix
5.15/6.1/6.6/6.12/6.18 in any case. The affected versions are only
Linux 7.0+.

Thanks,
Gao Xiang