binfmt_elf PT_INTERP gets EFAULT if no PROT_WRITE

[John Reiser]

In fs/binfmt_elf.c, routine load_elf_interp() calls padzero() for .bss
even if the PT_LOAD has no PROT_WRITE and no .bss.  This generates EFAULT.
One easy way to avoid trouble is that a PT_LOAD with no PROT_WRITE
should skip the .bss calculation entirely.

Here is a small test case.  (Yes, there are other, useful PT_INTERP
which have only .text and no .data/.bss.)

----- ptinterp.S
_start: .globl _start
        nop
        int3
-----
$ gcc -m32 -nostartfiles -nostdlib -o ptinterp ptinterp.S
$ gcc -m32 -Wl,--dynamic-linker=ptinterp -o hello hello.c
$ ./hello
Segmentation fault  # during execve() itself

After applying the patch:
$ ./hello
Trace trap  # user-mode execution after execve() finishes

Signed-off-by: John Reiser <jreiser@xxxxxxxxxxxx>


diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
index b7c1603..3b9a097 100644
--- a/fs/binfmt_elf.c
+++ b/fs/binfmt_elf.c
@@ -488,7 +488,7 @@ static unsigned long load_elf_interp(struct elfhdr *interp_elf_ex,
			 * keep track of the largest address we see for this.
			 */
			k = load_addr + eppnt->p_vaddr + eppnt->p_filesz;
-			if (k > elf_bss)
+			if (k > elf_bss && PROT_WRITE & elf_prot)
				elf_bss = k;

			/*
@@ -496,7 +496,7 @@ static unsigned long load_elf_interp(struct elfhdr *interp_elf_ex,
			 * elf_bss and last_bss is the bss section.
			 */
			k = load_addr + eppnt->p_memsz + eppnt->p_vaddr;
-			if (k > last_bss)
+			if (k > last_bss && PROT_WRITE & elf_prot)
				last_bss = k;
		}
	}

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