From: Chuck Ebbert <76306.1226@compuserve.com> 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 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 Signed-off-by: Andrew Morton --- arch/i386/mm/fault.c | 10 +++++----- 1 files changed, 5 insertions(+), 5 deletions(-) diff -puN arch/i386/mm/fault.c~i386-let-usermode-execute-the-enter arch/i386/mm/fault.c --- devel/arch/i386/mm/fault.c~i386-let-usermode-execute-the-enter 2006-05-18 01:29:58.000000000 -0700 +++ devel-akpm/arch/i386/mm/fault.c 2006-05-18 01:29:58.000000000 -0700 @@ -380,12 +380,12 @@ fastcall void __kprobes do_page_fault(st 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)) _