From 21528454f6dd18231ae20102f98aa8f51b6ec1b9 Mon Sep 17 00:00:00 2001
From: Chuck Ebbert <76306.1226@compuserve.com>
Date: Fri, 23 Jun 2006 02:04:23 -0700
Subject: [PATCH] i386: let usermode execute the "enter" instruction
The i386 page fault handler does not allow enough slack when checking for
userspace access below the current stack pointer. This prevents use of the
enter instruction by user code. Fix this by allowing enough slack for
"enter $65535,$31" to execute.
Problem reported by Tomasz Malesinski <tmal@mimuw.edu.pl>
Tested using this program, based on the original from Tomasz:
.file "ovflow.S"
.version "01.01"
gcc2_compiled.:
.section .rodata
.LC0:
.string "asdf\n"
.text
.align 4
.globl main
.type main,@function
main:
nest_level=0
.rept 30
enter $0,$nest_level
nest_level=nest_level+1
.endr
enter $65535,$30
enter $65535,$31
addl $-12,%esp
pushl $.LC0
call printf
addl $16,%esp
.L2:
.rept 32
leave
.endr
ret
.Lfe1:
.size main,.Lfe1-main
.ident "GCC: (GNU) 2.95.4 20011002 (Debian prerelease)"
Signed-off-by: Chuck Ebbert <76306.1226@compuserve.com>
Cc: Andi Kleen <ak@muc.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>
---
arch/i386/mm/fault.c | 10 +++++-----
1 file changed, 5 insertions(+), 5 deletions(-)
diff --git a/arch/i386/mm/fault.c b/arch/i386/mm/fault.c
index 7f0fcf219a..f38085ff3c 100644
--- a/arch/i386/mm/fault.c
+++ b/arch/i386/mm/fault.c
@@ -380,12 +380,12 @@ fastcall void __kprobes do_page_fault(struct pt_regs *regs,
goto bad_area;
if (error_code & 4) {
/*
- * accessing the stack below %esp is always a bug.
- * The "+ 32" is there due to some instructions (like
- * pusha) doing post-decrement on the stack and that
- * doesn't show up until later..
+ * Accessing the stack below %esp is always a bug.
+ * The large cushion allows instructions like enter
+ * and pusha to work. ("enter $65535,$31" pushes
+ * 32 pointers and then decrements %esp by 65535.)
*/
- if (address + 32 < regs->esp)
+ if (address + 65536 + 32 * sizeof(unsigned long) < regs->esp)
goto bad_area;
}
if (expand_stack(vma, address))
--
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