[RFC PATCH v1 0/1] binfmt_elf: seal address zero

[RFC PATCH v1 0/1] binfmt_elf: seal address zero

[jeffxu]

From: Jeff Xu <jeffxu@chromium.org>

In load_elf_binary as part of the execve(),  when the current
task’s personality has MMAP_PAGE_ZERO set, the kernel allocates
one page at address 0. According to the comment:

/* Why this, you ask???  Well SVr4 maps page 0 as read-only,
    and some applications "depend" upon this behavior.
    Since we do not have the power to recompile these, we
     emulate the SVr4 behavior. Sigh. */

At one point, Linus suggested removing this [1].

Sealing this is probably safe, the comment doesn’t say 
the app ever wanting to change the mapping to rwx. Sealing
also ensures that never happens.

[1] https://lore.kernel.org/lkml/CAHk-=whVa=nm_GW=NVfPHqcxDbWt4JjjK1YWb0cLjO4ZSGyiDA@mail.gmail.com/

Jeff Xu (1):
  binfmt_elf: mseal address zero

 fs/binfmt_elf.c    | 4 ++++
 include/linux/mm.h | 4 ++++
 mm/mseal.c         | 2 +-
 3 files changed, 9 insertions(+), 1 deletion(-)

-- 
2.46.0.rc1.232.g9752f9e123-goog

[RFC PATCH v1 1/1] binfmt_elf: mseal address zero

[jeffxu]

From: Jeff Xu <jeffxu@chromium.org>

Some legacy SVr4 apps might depend on page on address zero
to be readable, however I can't find a reason that the page
ever becomes writeable, so seal it.

If there is a compain, we can make this configurable.

Signed-off-by: Jeff Xu <jeffxu@chromium.org>
---
 fs/binfmt_elf.c    | 4 ++++
 include/linux/mm.h | 4 ++++
 mm/mseal.c         | 2 +-
 3 files changed, 9 insertions(+), 1 deletion(-)

diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
index 19fa49cd9907..e4d35d6f5d65 100644
--- a/fs/binfmt_elf.c
+++ b/fs/binfmt_elf.c
@@ -1314,6 +1314,10 @@ static int load_elf_binary(struct linux_binprm *bprm)
 		   emulate the SVr4 behavior. Sigh. */
 		error = vm_mmap(NULL, 0, PAGE_SIZE, PROT_READ | PROT_EXEC,
 				MAP_FIXED | MAP_PRIVATE, 0);
+
+#ifdef CONFIG_64BIT
+		do_mseal(0, PAGE_SIZE, 0);
+#endif
 	}
 
 	regs = current_pt_regs();
diff --git a/include/linux/mm.h b/include/linux/mm.h
index c4b238a20b76..b5fed60ddcd9 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -4201,4 +4201,8 @@ void vma_pgtable_walk_end(struct vm_area_struct *vma);
 
 int reserve_mem_find_by_name(const char *name, phys_addr_t *start, phys_addr_t *size);
 
+#ifdef CONFIG_64BIT
+int do_mseal(unsigned long start, size_t len_in, unsigned long flags);
+#endif
+
 #endif /* _LINUX_MM_H */
diff --git a/mm/mseal.c b/mm/mseal.c
index bf783bba8ed0..7a40a84569c8 100644
--- a/mm/mseal.c
+++ b/mm/mseal.c
@@ -248,7 +248,7 @@ static int apply_mm_seal(unsigned long start, unsigned long end)
  *
  *  unseal() is not supported.
  */
-static int do_mseal(unsigned long start, size_t len_in, unsigned long flags)
+int do_mseal(unsigned long start, size_t len_in, unsigned long flags)
 {
 	size_t len;
 	int ret = 0;
-- 
2.46.0.rc1.232.g9752f9e123-goog

Re: [RFC PATCH v1 1/1] binfmt_elf: mseal address zero

[Kees Cook]

On Thu, Aug 01, 2024 at 05:08:33PM +0000, jeffxu@chromium.org wrote:
> From: Jeff Xu <jeffxu@chromium.org>
> 
> Some legacy SVr4 apps might depend on page on address zero
> to be readable, however I can't find a reason that the page
> ever becomes writeable, so seal it.
> 
> If there is a compain, we can make this configurable.
> 
> Signed-off-by: Jeff Xu <jeffxu@chromium.org>
> ---
>  fs/binfmt_elf.c    | 4 ++++
>  include/linux/mm.h | 4 ++++
>  mm/mseal.c         | 2 +-
>  3 files changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
> index 19fa49cd9907..e4d35d6f5d65 100644
> --- a/fs/binfmt_elf.c
> +++ b/fs/binfmt_elf.c
> @@ -1314,6 +1314,10 @@ static int load_elf_binary(struct linux_binprm *bprm)
>  		   emulate the SVr4 behavior. Sigh. */
>  		error = vm_mmap(NULL, 0, PAGE_SIZE, PROT_READ | PROT_EXEC,
>  				MAP_FIXED | MAP_PRIVATE, 0);
> +
> +#ifdef CONFIG_64BIT
> +		do_mseal(0, PAGE_SIZE, 0);
> +#endif

Instead of wrapping this in #ifdefs, does it make more sense to adjust
the mm.h declaration instead, like this below...

>  	}
>  
>  	regs = current_pt_regs();
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index c4b238a20b76..b5fed60ddcd9 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -4201,4 +4201,8 @@ void vma_pgtable_walk_end(struct vm_area_struct *vma);
>  
>  int reserve_mem_find_by_name(const char *name, phys_addr_t *start, phys_addr_t *size);
>  
> +#ifdef CONFIG_64BIT
> +int do_mseal(unsigned long start, size_t len_in, unsigned long flags);

#else
static inline int do_mseal(unsigned long start, size_t len_in, unsigned long flags)
{
	return -ENOTSUPP;
}

> +#endif
> +
>  #endif /* _LINUX_MM_H */
> diff --git a/mm/mseal.c b/mm/mseal.c
> index bf783bba8ed0..7a40a84569c8 100644
> --- a/mm/mseal.c
> +++ b/mm/mseal.c
> @@ -248,7 +248,7 @@ static int apply_mm_seal(unsigned long start, unsigned long end)
>   *
>   *  unseal() is not supported.
>   */
> -static int do_mseal(unsigned long start, size_t len_in, unsigned long flags)
> +int do_mseal(unsigned long start, size_t len_in, unsigned long flags)
>  {
>  	size_t len;
>  	int ret = 0;
> -- 
> 2.46.0.rc1.232.g9752f9e123-goog
> 

And if it returns an error code, should we check it when used in
load_elf_binary()? (And if so, should the mm.h return 0 for non-64bit?)

-- 
Kees Cook

Re: [RFC PATCH v1 1/1] binfmt_elf: mseal address zero

[Jeff Xu]

On Mon, Aug 5, 2024 at 2:05 PM Kees Cook <kees@kernel.org> wrote:
>
> On Thu, Aug 01, 2024 at 05:08:33PM +0000, jeffxu@chromium.org wrote:
> > From: Jeff Xu <jeffxu@chromium.org>
> >
> > Some legacy SVr4 apps might depend on page on address zero
> > to be readable, however I can't find a reason that the page
> > ever becomes writeable, so seal it.
> >
> > If there is a compain, we can make this configurable.
> >
> > Signed-off-by: Jeff Xu <jeffxu@chromium.org>
> > ---
> >  fs/binfmt_elf.c    | 4 ++++
> >  include/linux/mm.h | 4 ++++
> >  mm/mseal.c         | 2 +-
> >  3 files changed, 9 insertions(+), 1 deletion(-)
> >
> > diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
> > index 19fa49cd9907..e4d35d6f5d65 100644
> > --- a/fs/binfmt_elf.c
> > +++ b/fs/binfmt_elf.c
> > @@ -1314,6 +1314,10 @@ static int load_elf_binary(struct linux_binprm *bprm)
> >                  emulate the SVr4 behavior. Sigh. */
> >               error = vm_mmap(NULL, 0, PAGE_SIZE, PROT_READ | PROT_EXEC,
> >                               MAP_FIXED | MAP_PRIVATE, 0);
> > +
> > +#ifdef CONFIG_64BIT
> > +             do_mseal(0, PAGE_SIZE, 0);
> > +#endif
>
> Instead of wrapping this in #ifdefs, does it make more sense to adjust
> the mm.h declaration instead, like this below...
>
Sure.

> >       }
> >
> >       regs = current_pt_regs();
> > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > index c4b238a20b76..b5fed60ddcd9 100644
> > --- a/include/linux/mm.h
> > +++ b/include/linux/mm.h
> > @@ -4201,4 +4201,8 @@ void vma_pgtable_walk_end(struct vm_area_struct *vma);
> >
> >  int reserve_mem_find_by_name(const char *name, phys_addr_t *start, phys_addr_t *size);
> >
> > +#ifdef CONFIG_64BIT
> > +int do_mseal(unsigned long start, size_t len_in, unsigned long flags);
>
> #else
> static inline int do_mseal(unsigned long start, size_t len_in, unsigned long flags)
> {
>         return -ENOTSUPP;
> }
>
OK.

> > +#endif
> > +
> >  #endif /* _LINUX_MM_H */
> > diff --git a/mm/mseal.c b/mm/mseal.c
> > index bf783bba8ed0..7a40a84569c8 100644
> > --- a/mm/mseal.c
> > +++ b/mm/mseal.c
> > @@ -248,7 +248,7 @@ static int apply_mm_seal(unsigned long start, unsigned long end)
> >   *
> >   *  unseal() is not supported.
> >   */
> > -static int do_mseal(unsigned long start, size_t len_in, unsigned long flags)
> > +int do_mseal(unsigned long start, size_t len_in, unsigned long flags)
> >  {
> >       size_t len;
> >       int ret = 0;
> > --
> > 2.46.0.rc1.232.g9752f9e123-goog
> >
>
> And if it returns an error code, should we check it when used in
> load_elf_binary()? (And if so, should the mm.h return 0 for non-64bit?)
>
It shouldn't fail. I can add pr_warning to handle the error case:
pr_warning("pid=%d, couldn't seal the page on address 0.\n",
task_pid_nr(current));

Thanks!
Best regards,
-Jeff


> --
> Kees Cook

Re: [RFC PATCH v1 0/1] binfmt_elf: seal address zero

[Kees Cook]

On Thu, Aug 01, 2024 at 05:08:32PM +0000, jeffxu@chromium.org wrote:
> From: Jeff Xu <jeffxu@chromium.org>
> 
> In load_elf_binary as part of the execve(),  when the current
> task’s personality has MMAP_PAGE_ZERO set, the kernel allocates
> one page at address 0. According to the comment:
> 
> /* Why this, you ask???  Well SVr4 maps page 0 as read-only,
>     and some applications "depend" upon this behavior.
>     Since we do not have the power to recompile these, we
>      emulate the SVr4 behavior. Sigh. */
> 
> At one point, Linus suggested removing this [1].

For users, I didn't find much in a Debian Code Search:
https://codesearch.debian.net/search?q=MMAP_PAGE_ZERO&literal=1&perpkg=1

I see rr uses it in testing, and some utils have it as an option, so I
think maybe just leave it supported.

> 
> Sealing this is probably safe, the comment doesn’t say 
> the app ever wanting to change the mapping to rwx. Sealing
> also ensures that never happens.

Yeah, this seems fine to me.

> 
> [1] https://lore.kernel.org/lkml/CAHk-=whVa=nm_GW=NVfPHqcxDbWt4JjjK1YWb0cLjO4ZSGyiDA@mail.gmail.com/
> 
> Jeff Xu (1):
>   binfmt_elf: mseal address zero
> 
>  fs/binfmt_elf.c    | 4 ++++
>  include/linux/mm.h | 4 ++++
>  mm/mseal.c         | 2 +-
>  3 files changed, 9 insertions(+), 1 deletion(-)
> 
> -- 
> 2.46.0.rc1.232.g9752f9e123-goog
> 

-- 
Kees Cook