Re: [PATCH stable 6.6] lib/buildid: Handle memfd_secret() files in build_id_parse()

[Greg Kroah-Hartman]

On Fri, Mar 14, 2025 at 02:40:39PM +0800, Chen Linxuan wrote:
> [ Upstream commit 5ac9b4e935dfc6af41eee2ddc21deb5c36507a9f ]
> 
> >>From memfd_secret(2) manpage:
> 
>   The memory areas backing the file created with memfd_secret(2) are
>   visible only to the processes that have access to the file descriptor.
>   The memory region is removed from the kernel page tables and only the
>   page tables of the processes holding the file descriptor map the
>   corresponding physical memory. (Thus, the pages in the region can't be
>   accessed by the kernel itself, so that, for example, pointers to the
>   region can't be passed to system calls.)
> 
> We need to handle this special case gracefully in build ID fetching
> code. Return -EFAULT whenever secretmem file is passed to build_id_parse()
> family of APIs. Original report and repro can be found in [0].
> 
>   [0] https://lore.kernel.org/bpf/ZwyG8Uro%2FSyTXAni@ly-workstation/
> 
> Fixes: de3ec364c3c3 ("lib/buildid: add single folio-based file reader abstraction")
> Reported-by: Yi Lai <yi1.lai@xxxxxxxxx>
> Suggested-by: Shakeel Butt <shakeel.butt@xxxxxxxxx>
> Signed-off-by: Andrii Nakryiko <andrii@xxxxxxxxxx>
> Signed-off-by: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
> Acked-by: Shakeel Butt <shakeel.butt@xxxxxxxxx>
> Link: https://lore.kernel.org/bpf/20241017175431.6183-A-hca@xxxxxxxxxxxxx
> Link: https://lore.kernel.org/bpf/20241017174713.2157873-1-andrii@xxxxxxxxxx
> [ Linxuan: perform an equivalent direct check without folio-based changes ]
> Fixes: 88a16a130933 ("perf: Add build id data in mmap2 event")
> Signed-off-by: Chen Linxuan <chenlinxuan@xxxxxxxxxx>
> ---
> 
> Some previous discussions can be found in the following links:
> https://lore.kernel.org/stable/05D0A9F7DE394601+20250311100555.310788-2-chenlinxuan@xxxxxxxxxx/
> 
> ---
>  lib/buildid.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/lib/buildid.c b/lib/buildid.c
> index 9fc46366597e..6249bd47fb0b 100644
> --- a/lib/buildid.c
> +++ b/lib/buildid.c
> @@ -5,6 +5,7 @@
>  #include <linux/elf.h>
>  #include <linux/kernel.h>
>  #include <linux/pagemap.h>
> +#include <linux/secretmem.h>
>  
>  #define BUILD_ID 3
>  
> @@ -157,6 +158,12 @@ int build_id_parse(struct vm_area_struct *vma, unsigned char *build_id,
>  	if (!vma->vm_file)
>  		return -EINVAL;
>  
> +#ifdef CONFIG_SECRETMEM
> +       /* reject secretmem folios created with memfd_secret() */
> +       if (vma->vm_file->f_mapping->a_ops == &secretmem_aops)
> +               return -EFAULT;
> +#endif

We really do not like #ifdef in .c files.  Why can't this use much the
same call it does in the original commit?  Just put that in the correct
.h file so that the #ifdef is not needed here, to make the backport
match much more cleanly.

thanks,

greg k-h