Re: [PATCH bpf] lib/buildid: handle memfd_secret() files in build_id_parse()

[Shakeel Butt <shakeel.butt@linux.dev>]

Ccing couple more folks who are doing similar work (ASI, guest_memfd)

Folks, what is the generic way to check if a given mapping has folios
unmapped from kernel address space?

On Mon, Oct 14, 2024 at 04:56:31PM GMT, Andrii Nakryiko wrote:
> 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 -EACCESS 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/
> 
> Reported-by: Yi Lai <yi1.lai@intel.com>
> Suggested-by: Shakeel Butt <shakeel.butt@linux.dev>
> Fixes: de3ec364c3c3 ("lib/buildid: add single folio-based file reader abstraction")
> Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
> ---
>  lib/buildid.c | 5 +++++
>  1 file changed, 5 insertions(+)
> 
> diff --git a/lib/buildid.c b/lib/buildid.c
> index 290641d92ac1..f0e6facf61c5 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
>  
> @@ -64,6 +65,10 @@ static int freader_get_folio(struct freader *r, loff_t file_off)
>  
>  	freader_put_folio(r);
>  
> +	/* reject secretmem folios created with memfd_secret() */
> +	if (secretmem_mapping(r->file->f_mapping))
> +		return -EACCES;
> +
>  	r->folio = filemap_get_folio(r->file->f_mapping, file_off >> PAGE_SHIFT);
>  
>  	/* if sleeping is allowed, wait for the page, if necessary */
> -- 
> 2.43.5
>