[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

@@ -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))