linux-hardening - Re: [PATCH v2] binfmt_elf: Allow .bss in any interp PT

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Date:   Fri, 11 Nov 2022 12:13:19 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Fangrui Song <maskray@...gle.com>
Cc:     Pedro Falcato <pedro.falcato@...il.com>,
        Rich Felker <dalias@...c.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Eric Biederman <ebiederm@...ssion.com>, sam@...too.org,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH v2] binfmt_elf: Allow .bss in any interp PT_LOAD

On Thu, Nov 10, 2022 at 11:42:34PM -0800, Fangrui Song wrote:
> (+ sam@...too.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@...c.org>
> > Link: https://maskray.me/blog/2022-11-05-lld-musl-powerpc64
> > Cc: Pedro Falcato <pedro.falcato@...il.com>
> > Cc: Fangrui Song <maskray@...gle.com>
> > Cc: Alexander Viro <viro@...iv.linux.org.uk>
> > Cc: Eric Biederman <ebiederm@...ssion.com>
> > Cc: linux-fsdevel@...r.kernel.org
> > Cc: linux-mm@...ck.org
> > Signed-off-by: Kees Cook <keescook@...omium.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.

Where is a.txt? Also, it'd be nice to have this without needing the
musl-gcc.

-- 
Kees Cook