Re: [PATCH v2] binfmt_elf: Allow .bss in any interp PT_LOAD
From: Fangrui Song
Date: Fri Nov 11 2022 - 15:27:46 EST
On 2022-11-11, Kees Cook wrote:
On Thu, Nov 10, 2022 at 11:42:34PM -0800, Fangrui Song wrote:
(+ sam@xxxxxxxxxx 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@xxxxxxxx>
> Link: https://maskray.me/blog/2022-11-05-lld-musl-powerpc64
> Cc: Pedro Falcato <pedro.falcato@xxxxxxxxx>
> Cc: Fangrui Song <maskray@xxxxxxxxxx>
> Cc: Alexander Viro <viro@xxxxxxxxxxxxxxxxxx>
> Cc: Eric Biederman <ebiederm@xxxxxxxxxxxx>
> Cc: linux-fsdevel@xxxxxxxxxxxxxxx
> Cc: linux-mm@xxxxxxxxx
> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> ---
> v2: I realized we need to retain the final padding call.
> v1: https://lore.kernel.org/linux-hardening/20221111055747.never.202-kees@xxxxxxxxxx/
> ---
> 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.
Where is a.txt? Also, it'd be nice to have this without needing the
musl-gcc.
Sorry for the unclear description. I rewrite it.
(`char nobits0[0] __attribute__((section(".nobits0")));` is not effective. It's SHT_PROGBITS and makes the output section SHT_PROGBITS.
The new example addresses the deficiency.)
Create some files. If you have split-file (a [test utility](https://llvm.org/docs/TestingGuide.html#extra-files) from llvm-project), you may place the following content into `a.txt`.
```text
#--- a.c
#include <assert.h>
#include <stdio.h>
extern const char toc[];
extern char nobits0[], nobits1[];
int main(void) {
assert(toc[4096-1] == 0);
for (int i = 0; i < 1024; i++) {
assert(nobits0[i] == 0);
nobits0[i] = 1;
}
for (int i = 0; i < 8192; i++) {
assert(nobits1[i] == 0);
nobits1[i] = 1;
}
puts("hello");
}
#--- toc.s
.globl toc, nobits0, nobits1
.section .toc,"aw",@nobits; toc: .space 4096
.section .ro0,"a"; .byte 255
.section .nobits0,"aw",@nobits; nobits0: .space 1024
.section .ro1,"a"; .byte 255
.section .nobits1,"aw",@nobits; nobits1: .space 8192
#--- a.lds
SECTIONS { .ro0 : {} .nobits0 : {} .ro1 : {} .nobits1 : {} } INSERT AFTER .bss;
```
Then run:
```sh
split-file a.txt a
path/to/musl-gcc -Wl,--dynamic-linker=/lib/libc.so a/a.c a/a.lds -o toy
```
Note: when a `SHT_NOBITS` section is followed by another section, the `SHT_NOBITS` section behaves as if it occupies the file offset range. This is because ld.lld does not implement a file size optimization.
For this simple example, using glibc based gcc works as well (musl provides __assert_fail and puts referenced by the executable):
gcc -Wl,--dynamic-linker=/lib/libc.so a/a.c a/a.lds -o toy