linux-kernel - binfmt_elf PT_INTERP gets EFAULT if no PROT

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Message-ID: <4AA6F653.1010007@bitwagon.com>
Date:	Tue, 08 Sep 2009 17:26:59 -0700
From:	John Reiser <jreiser@...wagon.com>
To:	Alexander Viro <viro@...iv.linux.org.uk>,
	James Morris <jmorris@...ei.org>,
	Roland McGrath <roland@...hat.com>,
	David Howells <dhowells@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>
CC:	linux-fsdevel@...r.kernel.org,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: binfmt_elf PT_INTERP gets EFAULT if no PROT_WRITE

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@...Wagon.com>


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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