fs/binfmt_elf: fix AT_PHDR for unusual ELF files

fs/binfmt_elf: fix AT_PHDR for unusual ELF files

Patch series "fs/binfmt_elf: Fix AT_PHDR for unusual ELF files", v4.

These patches fix a bug in AT_PHDR calculation.

We cannot calculate AT_PHDR as the sum of load_addr and exec->e_phoff.
This is because exec->e_phoff is the offset of PHDRs in the file and the
address of PHDRs in the memory may differ from it.  These patches fix the
bug by calculating the address of program headers from PT_LOADs directly.

This patch (of 2):

As pointed out in the bugzilla discussion, we cannot calculate AT_PHDR as
the sum of load_addr and exec->e_phoff.

: The AT_PHDR of ELF auxiliary vectors should point to the memory address
: of program header. But binfmt_elf.c calculates this address as follows:
:
: NEW_AUX_ENT(AT_PHDR, load_addr + exec->e_phoff);
:
: which is wrong since e_phoff is the file offset of program header and
: load_addr is the memory base address from PT_LOAD entry.
:
: The ld.so uses AT_PHDR as the memory address of program header. In normal
: case, since the e_phoff is usually 64 and in the first PT_LOAD region, it
: is the correct program header address.
:
: But if the address of program header isn't equal to the first PT_LOAD
: address + e_phoff (e.g.  Put the program header in other non-consecutive
: PT_LOAD region), ld.so will try to read program header from wrong address
: then crash or use incorrect program header.

This is because exec->e_phoff is the offset of PHDRs in the file and the
address of PHDRs in the memory may differ from it.  This patch fixes the
bug by calculating the address of program headers from PT_LOADs directly.

Link: https://lkml.kernel.org/r/20211212232414.1402199-1-akirakawata1@gmail.com
Link: https://bugzilla.kernel.org/show_bug.cgi?id=197921
Link: https://lkml.kernel.org/r/20211212232414.1402199-2-akirakawata1@gmail.com
Signed-off-by: Akira Kawata <akirakawata1@gmail.com>
Reported-by: kernel test robot <lkp@intel.com>
Acked-by: Kees Cook <keescook@chromium.org>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Cc: Eric Biederman <ebiederm@xmission.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>