Re: [RFC v2 6/9] Creation of "pagefault_handler_x86" LSM hook

From: Kees Cook
Date: Tue Jun 27 2017 - 19:07:54 EST

On Thu, Jun 15, 2017 at 9:42 AM, Salvatore Mesoraca
<s.mesoraca16@xxxxxxxxx> wrote:
> Creation of a new hook to let LSM modules handle user-space pagefaults on
> x86.
> It can be used to avoid segfaulting the originating process.
> If it's the case it can modify process registers before returning.
> This is not a security feature by itself, it's a way to soften some
> unwanted side-effects of restrictive security features.
> In particular this is used by S.A.R.A. can be used to implement what
> PaX call "trampoline emulation" that, in practice, allow for some specific
> code sequences to be executed even if they are in non executable memory.
> This may look like a bad thing at first, but you have to consider
> that:
> - This allows for strict memory restrictions (e.g. W^X) to stay on even
> when they should be turned off. And, even if this emulation
> makes those features less effective, it's still better than having
> them turned off completely.
> - The only code sequences emulated are trampolines used to make
> function calls. In many cases, when you have the chance to
> make arbitrary memory writes, you can already manipulate the
> control flow of the program by overwriting function pointers or
> return values. So, in many cases, the "trampoline emulation"
> doesn't introduce new exploit vectors.
> - It's a feature that can be turned on only if needed, on a per
> executable file basis.

Can this be made arch-agnostic? It seems a per-arch register-handling
routine would be needed, though. :(

-Kees

>
> Signed-off-by: Salvatore Mesoraca <s.mesoraca16@xxxxxxxxx>
> ---
> arch/x86/mm/fault.c | 6 ++++++
> include/linux/lsm_hooks.h | 9 +++++++++
> include/linux/security.h | 11 +++++++++++
> security/security.c | 11 +++++++++++
> 4 files changed, 37 insertions(+)
>
> diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
> index 8ad91a0..b75b81a 100644
> --- a/arch/x86/mm/fault.c
> +++ b/arch/x86/mm/fault.c
> @@ -15,6 +15,7 @@
> #include <linux/prefetch.h> /* prefetchw */
> #include <linux/context_tracking.h> /* exception_enter(), ... */
> #include <linux/uaccess.h> /* faulthandler_disabled() */
> +#include <linux/security.h> /* security_pagefault_handler */
>
> #include <asm/cpufeature.h> /* boot_cpu_has, ... */
> #include <asm/traps.h> /* dotraplinkage, ... */
> @@ -1358,6 +1359,11 @@ static inline bool smap_violation(int error_code, struct pt_regs *regs)
> local_irq_enable();
> }
>
> + if (unlikely(security_pagefault_handler_x86(regs,
> + error_code,
> + address)))
> + return;
> +
> perf_sw_event(PERF_COUNT_SW_PAGE_FAULTS, 1, regs, address);
>
> if (error_code & PF_WRITE)
> diff --git a/include/linux/lsm_hooks.h b/include/linux/lsm_hooks.h
> index 33dab16..da487e5 100644
> --- a/include/linux/lsm_hooks.h
> +++ b/include/linux/lsm_hooks.h
> @@ -488,6 +488,11 @@
> * @vmflags contains requested the vmflags.
> * Return 0 if the operation is allowed to continue otherwise return
> * the appropriate error code.
> + * @pagefault_handler_x86:
> + * Handle pagefaults on x86.
> + * @regs contains process' registers.
> + * @error_code contains error code for the pagefault.
> + * @address contains the address that caused the pagefault.
> * @file_lock:
> * Check permission before performing file locking operations.
> * Note: this hook mediates both flock and fcntl style locks.
> @@ -1483,6 +1488,9 @@
> int (*file_mprotect)(struct vm_area_struct *vma, unsigned long reqprot,
> unsigned long prot);
> int (*check_vmflags)(vm_flags_t vmflags);
> + int (*pagefault_handler_x86)(struct pt_regs *regs,
> + unsigned long error_code,
> + unsigned long address);
> int (*file_lock)(struct file *file, unsigned int cmd);
> int (*file_fcntl)(struct file *file, unsigned int cmd,
> unsigned long arg);
> @@ -1754,6 +1762,7 @@ struct security_hook_heads {
> struct list_head mmap_file;
> struct list_head file_mprotect;
> struct list_head check_vmflags;
> + struct list_head pagefault_handler_x86;
> struct list_head file_lock;
> struct list_head file_fcntl;
> struct list_head file_set_fowner;
> diff --git a/include/linux/security.h b/include/linux/security.h
> index 8701872..3b91999 100644
> --- a/include/linux/security.h
> +++ b/include/linux/security.h
> @@ -301,6 +301,9 @@ int security_mmap_file(struct file *file, unsigned long prot,
> int security_file_mprotect(struct vm_area_struct *vma, unsigned long reqprot,
> unsigned long prot);
> int security_check_vmflags(vm_flags_t vmflags);
> +int __maybe_unused security_pagefault_handler_x86(struct pt_regs *regs,
> + unsigned long error_code,
> + unsigned long address);
> int security_file_lock(struct file *file, unsigned int cmd);
> int security_file_fcntl(struct file *file, unsigned int cmd, unsigned long arg);
> void security_file_set_fowner(struct file *file);
> @@ -829,6 +832,14 @@ static inline int security_check_vmflags(vm_flags_t vmflags)
> return 0;
> }
>
> +static inline int __maybe_unused security_pagefault_handler_x86(
> + struct pt_regs *regs,
> + unsigned long error_code,
> + unsigned long address)
> +{
> + return 0;
> +}
> +
> static inline int security_file_lock(struct file *file, unsigned int cmd)
> {
> return 0;
> diff --git a/security/security.c b/security/security.c
> index 7e45846..f7df697 100644
> --- a/security/security.c
> +++ b/security/security.c
> @@ -905,6 +905,17 @@ int security_check_vmflags(vm_flags_t vmflags)
> return call_int_hook(check_vmflags, 0, vmflags);
> }
>
> +int __maybe_unused security_pagefault_handler_x86(struct pt_regs *regs,
> + unsigned long error_code,
> + unsigned long address)
> +{
> + return call_int_hook(pagefault_handler_x86,
> + 0,
> + regs,
> + error_code,
> + address);
> +}
> +
> int security_file_lock(struct file *file, unsigned int cmd)
> {
> return call_int_hook(file_lock, 0, file, cmd);
> --
> 1.9.1
>



--
Kees Cook
Pixel Security