linux-hardening - Re: [PATCH v4 0/6] binfmt_elf: Support segments with 0 filesz and misaligned starts

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Date: Fri, 29 Sep 2023 13:33:50 +0200 (CEST)
From: Sebastian Ott <sebott@...hat.com>
To: Kees Cook <keescook@...omium.org>
cc: Eric Biederman <ebiederm@...ssion.com>, 
    Thomas Weißschuh <linux@...ssschuh.net>, 
    Pedro Falcato <pedro.falcato@...il.com>, Al Viro <viro@...iv.linux.org.uk>, 
    Christian Brauner <brauner@...nel.org>, 
    Andrew Morton <akpm@...ux-foundation.org>, linux-kernel@...r.kernel.org, 
    linux-fsdevel@...r.kernel.org, linux-mm@...ck.org, 
    linux-hardening@...r.kernel.org
Subject: Re: [PATCH v4 0/6] binfmt_elf: Support segments with 0 filesz and
 misaligned starts

Hello Kees,

On Thu, 28 Sep 2023, Kees Cook wrote:
> This is the continuation of the work Eric started for handling
> "p_memsz > p_filesz" in arbitrary segments (rather than just the last,
> BSS, segment). I've added the suggested changes:
>
> - drop unused "elf_bss" variable
> - refactor load_elf_interp() to use elf_load()
> - refactor load_elf_library() to use elf_load()
> - report padzero() errors when PROT_WRITE is present
> - drop vm_brk()

While I was debugging the initial issue I stumbled over the following
- care to take it as part of this series?

----->8
[PATCH] mm: vm_brk_flags don't bail out while holding lock

Calling vm_brk_flags() with flags set other than VM_EXEC
will exit the function without releasing the mmap_write_lock.

Just do the sanity check before the lock is acquired. This
doesn't fix an actual issue since no caller sets a flag other
than VM_EXEC.

Cc: Andrew Morton <akpm@...ux-foundation.org>
Cc: linux-mm@...ck.org
Signed-off-by: Sebastian Ott <sebott@...hat.com>
---
  mm/mmap.c | 6 +++---
  1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/mm/mmap.c b/mm/mmap.c
index b56a7f0c9f85..7ed286662839 100644
--- a/mm/mmap.c
+++ b/mm/mmap.c
@@ -3143,13 +3143,13 @@ int vm_brk_flags(unsigned long addr, unsigned long request, unsigned long flags)
  	if (!len)
  		return 0;

-	if (mmap_write_lock_killable(mm))
-		return -EINTR;
-
  	/* Until we need other flags, refuse anything except VM_EXEC. */
  	if ((flags & (~VM_EXEC)) != 0)
  		return -EINVAL;

+	if (mmap_write_lock_killable(mm))
+		return -EINTR;
+
  	ret = check_brk_limits(addr, len);
  	if (ret)
  		goto limits_failed;
-- 
2.41.0