linux-kernel - Re: binfmt_elf PT_INTERP gets EFAULT if no PROT

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Date:	Tue,  8 Sep 2009 19:49:40 -0700 (PDT)
From:	Roland McGrath <roland@...hat.com>
To:	John Reiser <jreiser@...wagon.com>
Cc:	Alexander Viro <viro@...iv.linux.org.uk>,
	James Morris <jmorris@...ei.org>,
	David Howells <dhowells@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-fsdevel@...r.kernel.org,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: binfmt_elf PT_INTERP gets EFAULT if no PROT_WRITE

Good catch, John.  But I prefer a different fix.  

I've reproduced the bug with the test case John included,
and verified that my change fixes it too.


Thanks,
Roland

---
[PATCH] binfmt_elf: fix PT_INTERP bss handling

In fs/binfmt_elf.c, load_elf_interp() calls padzero() for .bss even if
the PT_LOAD has no PROT_WRITE and no .bss.  This generates EFAULT.

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

If the ELF headers are actually self-inconsistent, then dying is fine.
But having no PROT_WRITE segment is perfectly normal and correct if
there is no segment with p_memsz > p_filesz (i.e. bss).  John Reiser
suggested checking for PROT_WRITE in the bss logic.  I think it makes
most sense to simply apply the bss logic only when there is bss.

This patch looks less trivial than it is due to some reindentation.
It just moves the "if (last_bss > elf_bss) {" test up to include the
partial-page bss logic as well as the more-pages bss logic.

Reported-by: John Reiser <jreiser@...wagon.com>
Signed-off-by: Roland McGrath <roland@...hat.com>
---
 fs/binfmt_elf.c |   28 ++++++++++++++--------------
 1 files changed, 14 insertions(+), 14 deletions(-)

diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
index b7c1603..7c1e65d 100644
--- a/fs/binfmt_elf.c
+++ b/fs/binfmt_elf.c
@@ -501,22 +501,22 @@ static unsigned long load_elf_interp(struct elfhdr *interp_elf_ex,
 		}
 	}
 
-	/*
-	 * Now fill out the bss section.  First pad the last page up
-	 * to the page boundary, and then perform a mmap to make sure
-	 * that there are zero-mapped pages up to and including the 
-	 * last bss page.
-	 */
-	if (padzero(elf_bss)) {
-		error = -EFAULT;
-		goto out_close;
-	}
+	if (last_bss > elf_bss) {
+		/*
+		 * Now fill out the bss section.  First pad the last page up
+		 * to the page boundary, and then perform a mmap to make sure
+		 * that there are zero-mapped pages up to and including the
+		 * last bss page.
+		 */
+		if (padzero(elf_bss)) {
+			error = -EFAULT;
+			goto out_close;
+		}
 
-	/* What we have mapped so far */
-	elf_bss = ELF_PAGESTART(elf_bss + ELF_MIN_ALIGN - 1);
+		/* What we have mapped so far */
+		elf_bss = ELF_PAGESTART(elf_bss + ELF_MIN_ALIGN - 1);
 
-	/* Map the last of the bss segment */
-	if (last_bss > elf_bss) {
+		/* Map the last of the bss segment */
 		down_write(&current->mm->mmap_sem);
 		error = do_brk(elf_bss, last_bss - elf_bss);
 		up_write(&current->mm->mmap_sem);
--
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