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Date: Tue, 12 Jul 2011 00:14:32 +0200
From: Borislav Petkov <bp@...en8.de>
To: Andy Lutomirski <luto@....EDU>
Cc: x86@...nel.org, linux-kernel@...r.kernel.org,
Ingo Molnar <mingo@...e.hu>, John Stultz <johnstul@...ibm.com>,
Borislav Petkov <bp@...64.org>
Subject: Re: [PATCH v2 1/8] x86-64: Improve vsyscall emulation CS and RIP
handling
On Sat, Jul 09, 2011 at 11:22:08PM -0400, Andy Lutomirski wrote:
> Two fixes here:
> - Send SIGSEGV if called from compat code or with a funny CS.
> - Don't BUG on impossible addresses.
>
> This patch also removes an unused variable.
>
> Signed-off-by: Andy Lutomirski <luto@....edu>
> ---
> arch/x86/include/asm/vsyscall.h | 12 --------
> arch/x86/kernel/vsyscall_64.c | 59 +++++++++++++++++++++++++--------------
> 2 files changed, 38 insertions(+), 33 deletions(-)
>
> diff --git a/arch/x86/include/asm/vsyscall.h b/arch/x86/include/asm/vsyscall.h
> index bb710cb..d555973 100644
> --- a/arch/x86/include/asm/vsyscall.h
> +++ b/arch/x86/include/asm/vsyscall.h
> @@ -31,18 +31,6 @@ extern struct timezone sys_tz;
>
> extern void map_vsyscall(void);
>
> -/* Emulation */
> -
> -static inline bool is_vsyscall_entry(unsigned long addr)
> -{
> - return (addr & ~0xC00UL) == VSYSCALL_START;
> -}
> -
> -static inline int vsyscall_entry_nr(unsigned long addr)
> -{
> - return (addr & 0xC00UL) >> 10;
> -}
> -
> #endif /* __KERNEL__ */
>
> #endif /* _ASM_X86_VSYSCALL_H */
> diff --git a/arch/x86/kernel/vsyscall_64.c b/arch/x86/kernel/vsyscall_64.c
> index 10cd8ac..6d14848 100644
> --- a/arch/x86/kernel/vsyscall_64.c
> +++ b/arch/x86/kernel/vsyscall_64.c
> @@ -38,6 +38,7 @@
>
> #include <asm/vsyscall.h>
> #include <asm/pgtable.h>
> +#include <asm/compat.h>
> #include <asm/page.h>
> #include <asm/unistd.h>
> #include <asm/fixmap.h>
> @@ -97,33 +98,60 @@ static void warn_bad_vsyscall(const char *level, struct pt_regs *regs,
>
> tsk = current;
>
> - printk("%s%s[%d] %s ip:%lx sp:%lx ax:%lx si:%lx di:%lx\n",
> + printk("%s%s[%d] %s ip:%lx cs:%lx sp:%lx ax:%lx si:%lx di:%lx\n",
> level, tsk->comm, task_pid_nr(tsk),
> - message, regs->ip - 2, regs->sp, regs->ax, regs->si, regs->di);
> + message, regs->ip - 2, regs->cs,
> + regs->sp, regs->ax, regs->si, regs->di);
> +}
> +
> +static int addr_to_vsyscall_nr(unsigned *vsyscall_nr, unsigned long addr)
> +{
> + if ((addr & ~0xC00UL) != VSYSCALL_START)
> + return -EINVAL;
> +
> + *vsyscall_nr = (addr & 0xC00UL) >> 10;
> + if (*vsyscall_nr >= 3)
> + return -EINVAL;
> +
> + return 0;
> }
I'm wondering: why don't you make this function return negative value on
error, i.e. -EINVAL and the vsyscall number on success so that you can
get rid of returning it through the arg pointer?
Then at the callsite you can do:
vsyscall_nr = addr_to_vsyscall_nr(addr);
if (vsyscall_nr < 0)
warn_bad_vsyscall(...)
?
>
> void dotraplinkage do_emulate_vsyscall(struct pt_regs *regs, long error_code)
> {
> - const char *vsyscall_name;
> struct task_struct *tsk;
> unsigned long caller;
> - int vsyscall_nr;
> + unsigned vsyscall_nr;
> long ret;
>
> - /* Kernel code must never get here. */
> - BUG_ON(!user_mode(regs));
> -
> local_irq_enable();
>
> /*
> + * Real 64-bit user mode code has cs == __USER_CS. Anything else
> + * is bogus.
> + */
> + if (regs->cs != __USER_CS) {
> + /*
> + * If we trapped from kernel mode, we might as well OOPS now
> + * instead of returning to some random address and OOPSing
> + * then.
> + */
> + BUG_ON(!user_mode(regs));
> +
> + /* Compat mode and non-compat 32-bit CS should both segfault. */
> + warn_bad_vsyscall(KERN_WARNING, regs,
> + "illegal int 0xcc from 32-bit mode");
> + goto sigsegv;
> + }
> +
> + /*
> * x86-ism here: regs->ip points to the instruction after the int 0xcc,
> * and int 0xcc is two bytes long.
> */
> - if (!is_vsyscall_entry(regs->ip - 2)) {
> - warn_bad_vsyscall(KERN_WARNING, regs, "illegal int 0xcc (exploit attempt?)");
> + if (addr_to_vsyscall_nr(&vsyscall_nr, regs->ip - 2) != 0) {
> + warn_bad_vsyscall(KERN_WARNING, regs,
> + "illegal int 0xcc (exploit attempt?)");
> goto sigsegv;
> }
> - vsyscall_nr = vsyscall_entry_nr(regs->ip - 2);
>
> if (get_user(caller, (unsigned long __user *)regs->sp) != 0) {
> warn_bad_vsyscall(KERN_WARNING, regs, "int 0xcc with bad stack (exploit attempt?)");
> @@ -136,31 +164,20 @@ void dotraplinkage do_emulate_vsyscall(struct pt_regs *regs, long error_code)
>
> switch (vsyscall_nr) {
> case 0:
> - vsyscall_name = "gettimeofday";
> ret = sys_gettimeofday(
> (struct timeval __user *)regs->di,
> (struct timezone __user *)regs->si);
> break;
>
> case 1:
> - vsyscall_name = "time";
> ret = sys_time((time_t __user *)regs->di);
> break;
>
> case 2:
> - vsyscall_name = "getcpu";
> ret = sys_getcpu((unsigned __user *)regs->di,
> (unsigned __user *)regs->si,
> 0);
> break;
> -
> - default:
> - /*
> - * If we get here, then vsyscall_nr indicates that int 0xcc
> - * happened at an address in the vsyscall page that doesn't
> - * contain int 0xcc. That can't happen.
> - */
> - BUG();
> }
>
> if (ret == -EFAULT) {
> --
> 1.7.6
Thanks.
--
Regards/Gruss,
Boris.
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