kernel/kexec_file.c: remove checks in kexec_purgatory_load

Before the purgatory is loaded several checks are done whether the ELF
file in kexec_purgatory is valid or not.  These checks are incomplete.
For example they don't check for the total size of the sections defined
in the section header table or if the entry point actually points into
the purgatory.

On the other hand the purgatory, although an ELF file on its own, is
part of the kernel.  Thus not trusting the purgatory means not trusting
the kernel build itself.

So remove all validity checks on the purgatory and just trust the kernel
build.

Link: http://lkml.kernel.org/r/20180321112751.22196-3-prudo@linux.vnet.ibm.com
Signed-off-by: Philipp Rudo <prudo@linux.vnet.ibm.com>
Acked-by: Dave Young <dyoung@redhat.com>
Cc: AKASHI Takahiro <takahiro.akashi@linaro.org>
Cc: Eric Biederman <ebiederm@xmission.com>
Cc: Heiko Carstens <heiko.carstens@de.ibm.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Martin Schwidefsky <schwidefsky@de.ibm.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Thiago Jung Bauermann <bauerman@linux.vnet.ibm.com>
Cc: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/kexec_file.c

@@ -941,22 +941,8 @@ int kexec_load_purgatory(struct kimage *image, unsigned long min,
 	if (kexec_purgatory_size <= 0)
 		return -EINVAL;
 
-	if (kexec_purgatory_size < sizeof(Elf_Ehdr))
-		return -ENOEXEC;
-
 	pi->ehdr = (Elf_Ehdr *)kexec_purgatory;
 
-	if (memcmp(pi->ehdr->e_ident, ELFMAG, SELFMAG) != 0
-	    || pi->ehdr->e_type != ET_REL
-	    || !elf_check_arch(pi->ehdr)
-	    || pi->ehdr->e_shentsize != sizeof(Elf_Shdr))
-		return -ENOEXEC;
-
-	if (pi->ehdr->e_shoff >= kexec_purgatory_size
-	    || (pi->ehdr->e_shnum * sizeof(Elf_Shdr) >
-	    kexec_purgatory_size - pi->ehdr->e_shoff))
-		return -ENOEXEC;
-
 	ret = __kexec_load_purgatory(image, min, max, top_down);
 	if (ret)
 		return ret;