linux-kernel - [PATCH] binfmt_elf: Do not move brk for INTERP-less ET

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Date:   Thu, 26 Sep 2019 10:15:25 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Richard Kojedzinszky <richard@...edz.in>,
        linux-kernel@...r.kernel.org, Ali Saidi <alisaidi@...zon.com>,
        Guenter Roeck <linux@...ck-us.net>,
        Michal Hocko <mhocko@...e.com>,
        Matthew Wilcox <willy@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Jann Horn <jannh@...gle.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: [PATCH] binfmt_elf: Do not move brk for INTERP-less ET_EXEC

When brk was moved for binaries without an interpreter, it should have
been limited to ET_DYN only. In other words, the special case was an
ET_DYN that lacks an INTERP, not just an executable that lacks INTERP.
The bug manifested for giant static executables, where the brk would end
up in the middle of the text area on 32-bit architectures.

Reported-by: Richard Kojedzinszky <richard@...edz.in>
Fixes: bbdc6076d2e5 ("binfmt_elf: move brk out of mmap when doing direct loader exec")
Cc: stable@...r.kernel.org
Signed-off-by: Kees Cook <keescook@...omium.org>
---
Richard, are you able to test this? I'm able to run the gitea binary
with this change, and my INTERP-less ET_DYN tests (from the original
bug) continue to pass as well.
---
 fs/binfmt_elf.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/fs/binfmt_elf.c b/fs/binfmt_elf.c
index cec3b4146440..ad4c6b1d5074 100644
--- a/fs/binfmt_elf.c
+++ b/fs/binfmt_elf.c
@@ -1121,7 +1121,8 @@ static int load_elf_binary(struct linux_binprm *bprm)
 		 * (since it grows up, and may collide early with the stack
 		 * growing down), and into the unused ELF_ET_DYN_BASE region.
 		 */
-		if (IS_ENABLED(CONFIG_ARCH_HAS_ELF_RANDOMIZE) && !interpreter)
+		if (IS_ENABLED(CONFIG_ARCH_HAS_ELF_RANDOMIZE) &&
+		    loc->elf_ex.e_type == ET_DYN && !interpreter)
 			current->mm->brk = current->mm->start_brk =
 				ELF_ET_DYN_BASE;
 
-- 
2.17.1


-- 
Kees Cook