(+ 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.
