Re: [PATCH v2] binfmt_elf: Allow .bss in any interp PT_LOAD

[Fangrui Song <maskray@google.com>]

(+ sam@gentoo.org from Pedro Falcato's patch)

On 2022-11-10, Kees Cook wrote:
>Traditionally, only the final PT_LOAD for load_elf_interp() supported
>having p_memsz > p_filesz. Recently, lld's construction of musl's
>libc.so on PowerPC64 started having two PT_LOAD program headers with
>p_memsz > p_filesz.
>
>As the least invasive change possible, check for p_memsz > p_filesz for
>each PT_LOAD in load_elf_interp.
>
>Reported-by: Rich Felker <dalias@libc.org>
>Link: https://maskray.me/blog/2022-11-05-lld-musl-powerpc64
>Cc: Pedro Falcato <pedro.falcato@gmail.com>
>Cc: Fangrui Song <maskray@google.com>
>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
>Signed-off-by: Kees Cook <keescook@chromium.org>
>---
>v2: I realized we need to retain the final padding call.
>v1: https://lore.kernel.org/linux-hardening/20221111055747.never.202-kees@kernel.org/
>---
> fs/binfmt_elf.c | 18 ++++++++++++++----
> 1 file changed, 14 insertions(+), 4 deletions(-)
>
>diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
>index 528e2ac8931f..0a24bbbef1d6 100644
>--- a/fs/binfmt_elf.c
>+++ b/fs/binfmt_elf.c
>@@ -673,15 +673,25 @@ static unsigned long load_elf_interp(struct elfhdr *interp_elf_ex,
> 				last_bss = k;
> 				bss_prot = elf_prot;
> 			}
>+
>+			/*
>+			 * Clear any p_memsz > p_filesz area up to the end
>+			 * of the page to wipe anything left over from the
>+			 * loaded file contents.
>+			 */
>+			if (last_bss > elf_bss && padzero(elf_bss))

Missing {

But after fixing this, I get a musl ld.so error.

>+				error = -EFAULT;
>+				goto out;
>+			}
> 		}
> 	}
>
> 	/*
>-	 * Now fill out the bss section: first pad the last page from
>-	 * the file up to the page boundary, and zero it from elf_bss
>-	 * up to the end of the page.
>+	 * Finally, pad the last page from the file up to the page boundary,
>+	 * and zero it from elf_bss up to the end of the page, if this did
>+	 * not already happen with the last PT_LOAD.
> 	 */
>-	if (padzero(elf_bss)) {
>+	if (last_bss == elf_bss && padzero(elf_bss)) {
> 		error = -EFAULT;
> 		goto out;
> 	}
>-- 
>2.34.1
>

I added a new section to https://maskray.me/blog/2022-11-05-lld-musl-powerpc64
Copying here:

To test that the kernel ELF loader can handle more RW `PT_LOAD` program headers, we can create an executable with more RW `PT_LOAD` program headers with `p_filesz < p_memsz`.
We can place a read-only section after `.bss` followed by a `SHT_NOBITS` `SHF_ALLOC|SHF_WRITE` section. The read-only section will form a read-only `PT_LOAD` while the RW section will form a RW `PT_LOAD`.

```text
#--- a.c
#include <assert.h>
#include <stdio.h>

extern const char toc[];
char nobits0[0] __attribute__((section(".nobits0")));
char nobits1[0] __attribute__((section(".nobits1")));

int main(void) {
   assert(toc[4096-1] == 0);
   for (int i = 0; i < 1024; i++)
     assert(nobits0[i] == 0);
   nobits0[0] = nobits0[1024-1] = 1;
   for (int i = 0; i < 4096; i++)
     assert(nobits1[i] == 0);
   nobits1[0] = nobits1[4096-1] = 1;

   puts("hello");
}

#--- toc.s
.section .toc,"aw",@nobits
.globl toc
toc:
.space 4096

.section .ro0,"a"; .byte 255
.section .nobits0,"aw",@nobits; .space 1024
.section .ro1,"a"; .byte 255
.section .nobits1,"aw",@nobits; .space 4096

#--- a.lds
SECTIONS { .ro0 : {} .nobits0 : {} .ro1 : {} .nobits1 : {} } INSERT AFTER .bss;
```

```sh
split-file a.txt a
path/to/musl-gcc -Wl,--dynamic-linker=/lib/libc.so a/a.c a/a.lds -o toy
```

split-file is a utility in llvm-project.