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

[Salvatore Mesoraca <s.mesoraca16@gmail.com>]

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.

Signed-off-by: Salvatore Mesoraca <s.mesoraca16@gmail.com>
---
 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