Re: [PATCH 5.16 v2] binfmt_elf: Avoid total_mapping_size for ET_EXEC

[Kees Cook <keescook@chromium.org>]

On Mon, Feb 28, 2022 at 05:14:26PM -0500, matoro wrote:
> On 2022-02-28 15:55, Kees Cook wrote:
> > Partially revert commit 5f501d555653 ("binfmt_elf: reintroduce using
> > MAP_FIXED_NOREPLACE").
> > 
> > At least ia64 has ET_EXEC PT_LOAD segments that are not virtual-address
> > contiguous (but _are_ file-offset contiguous). This would result in
> > giant mapping attempts to cover the entire span, including the virtual
> > address range hole. Disable total_mapping_size for ET_EXEC, which
> > reduces the MAP_FIXED_NOREPLACE coverage to only the first PT_LOAD:
> > 
> > $ readelf -lW /usr/bin/gcc
> > ...
> > Program Headers:
> >   Type Offset   VirtAddr           PhysAddr           FileSiz  MemSiz
> > ...
> > ...
> >   LOAD 0x000000 0x4000000000000000 0x4000000000000000 0x00b5a0 0x00b5a0
> > ...
> >   LOAD 0x00b5a0 0x600000000000b5a0 0x600000000000b5a0 0x0005ac 0x000710
> > ...
> > ...
> >        ^^^^^^^^ ^^^^^^^^^^^^^^^^^^                    ^^^^^^^^ ^^^^^^^^
> > 
> > File offset range     : 0x000000-0x00bb4c
> > 			0x00bb4c bytes
> > 
> > Virtual address range : 0x4000000000000000-0x600000000000bcb0
> > 			0x200000000000bcb0 bytes
> > 
> > Ironically, this is the reverse of the problem that originally caused
> > problems with ET_EXEC and MAP_FIXED_NOREPLACE: overlaps. This problem is
> > with holes. Future work could restore full coverage if load_elf_binary()
> > were to perform mappings in a separate phase from the loading (where
> > it could resolve both overlaps and holes).
> > 
> > Cc: Alexander Viro <viro@zeniv.linux.org.uk>
> > Cc: Eric Biederman <ebiederm@xmission.com>
> > Cc: linux-fsdevel@vger.kernel.org
> > Cc: linux-mm@kvack.org
> > Reported-by: matoro <matoro_mailinglist_kernel@matoro.tk>
> > Reported-by: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>
> > Fixes: 5f501d555653 ("binfmt_elf: reintroduce using
> > MAP_FIXED_NOREPLACE")
> > Link:
> > https://lore.kernel.org/r/a3edd529-c42d-3b09-135c-7e98a15b150f@leemhuis.info
> > Cc: stable@vger.kernel.org
> > Signed-off-by: Kees Cook <keescook@chromium.org>
> > ---
> > Here's the v5.16 backport.
> > ---
> >  fs/binfmt_elf.c | 25 ++++++++++++++++++-------
> >  1 file changed, 18 insertions(+), 7 deletions(-)
> > 
> > diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
> > index f8c7f26f1fbb..911a9e7044f4 100644
> > --- a/fs/binfmt_elf.c
> > +++ b/fs/binfmt_elf.c
> > @@ -1135,14 +1135,25 @@ static int load_elf_binary(struct linux_binprm
> > *bprm)
> >  			 * is then page aligned.
> >  			 */
> >  			load_bias = ELF_PAGESTART(load_bias - vaddr);
> > -		}
> > 
> > -		/*
> > -		 * Calculate the entire size of the ELF mapping (total_size).
> > -		 * (Note that load_addr_set is set to true later once the
> > -		 * initial mapping is performed.)
> > -		 */
> > -		if (!load_addr_set) {
> > +			/*
> > +			 * Calculate the entire size of the ELF mapping
> > +			 * (total_size), used for the initial mapping,
> > +			 * due to first_pt_load which is set to false later
> > +			 * once the initial mapping is performed.
> > +			 *
> > +			 * Note that this is only sensible when the LOAD
> > +			 * segments are contiguous (or overlapping). If
> > +			 * used for LOADs that are far apart, this would
> > +			 * cause the holes between LOADs to be mapped,
> > +			 * running the risk of having the mapping fail,
> > +			 * as it would be larger than the ELF file itself.
> > +			 *
> > +			 * As a result, only ET_DYN does this, since
> > +			 * some ET_EXEC (e.g. ia64) may have virtual
> > +			 * memory holes between LOADs.
> > +			 *
> > +			 */
> >  			total_size = total_mapping_size(elf_phdata,
> >  							elf_ex->e_phnum);
> >  			if (!total_size) {
> 
> This does the trick!  Thank you so much!!

Excellent; thank you for testing! I'll send this to Linus shortly.

-- 
Kees Cook