Messages in this thread Patch in this message | ![/](/images/icornerl.gif) | | From | Andy Lutomirski <> | Subject | [PATCH 3/9] x86: Add user_mode_ignore_vm86 | Date | Wed, 18 Mar 2015 18:33:29 -0700 |
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user_mode is dangerous and user_mode_vm has a confusing name. Add user_mode_ignore_vm86 (equivalent to current user_mode). We'll change the small number of legitimate users of user_mode to user_mode_ignore_vm86.
Inspired by grsec, although this works rather differently.
Signed-off-by: Andy Lutomirski <luto@kernel.org> --- arch/x86/include/asm/ptrace.h | 17 +++++++++++++++++ 1 file changed, 17 insertions(+)
diff --git a/arch/x86/include/asm/ptrace.h b/arch/x86/include/asm/ptrace.h index 74bb2e0f3030..a60c59e977cc 100644 --- a/arch/x86/include/asm/ptrace.h +++ b/arch/x86/include/asm/ptrace.h @@ -121,6 +121,23 @@ static inline int user_mode_vm(struct pt_regs *regs) #endif } +/* + * This is the fastest way to check whether regs come from user space. + * It is unsafe if regs might come from vm86 mode, though -- in vm86 + * mode, all bits of CS and SS are completely under the user's control. + * The CPU considers vm86 mode to be CPL 3 regardless of CS and SS. + * + * Do NOT use this function unless you have already ruled out the + * possibility that regs came from vm86 mode. + * + * We check for RPL != 0 instead of RPL == 3 because we don't use rings + * 1 or 2 and this is more efficient. + */ +static inline int user_mode_ignore_vm86(struct pt_regs *regs) +{ + return (regs->cs & SEGMENT_RPL_MASK) != 0; +} + static inline int v8086_mode(struct pt_regs *regs) { #ifdef CONFIG_X86_32 -- 2.3.0
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