From: Dave Hansen dave.hansen@linux.intel.com
I got a bug report that the following code (roughly) was causing a SIGSEGV:
mprotect(ptr, size, PROT_EXEC); mprotect(ptr, size, PROT_NONE); mprotect(ptr, size, PROT_READ); *ptr = 100;
The problem is hit when the mprotect(PROT_EXEC) is implicitly assigned a protection key to the VMA, and made that key ACCESS_DENY|WRITE_DENY. The PROT_NONE mprotect() failed to remove the protection key, and the PROT_NONE-> PROT_READ left the PTE usable, but the pkey still in place and left the memory inaccessible.
To fix this, we ensure that we always "override" the pkee at mprotect() if the VMA does not have execute-only permissions, but the VMA has the execute-only pkey.
We had a check for PROT_READ/WRITE, but it did not work for PROT_NONE. This entirely removes the PROT_* checks, which ensures that PROT_NONE now works.
Signed-off-by: Dave Hansen dave.hansen@linux.intel.com Fixes: 62b5f7d013f ("mm/core, x86/mm/pkeys: Add execute-only protection keys support") Reported-by: Shakeel Butt shakeelb@google.com Cc: stable@vger.kernel.org Cc: Ram Pai linuxram@us.ibm.com Cc: Thomas Gleixner tglx@linutronix.de Cc: Dave Hansen dave.hansen@intel.com Cc: Michael Ellermen mpe@ellerman.id.au Cc: Ingo Molnar mingo@kernel.org Cc: Andrew Morton akpm@linux-foundation.org Cc: Shuah Khan shuah@kernel.org ---
b/arch/x86/include/asm/pkeys.h | 12 +++++++++++- b/arch/x86/mm/pkeys.c | 21 +++++++++++---------- 2 files changed, 22 insertions(+), 11 deletions(-)
diff -puN arch/x86/include/asm/pkeys.h~pkeys-abandon-exec-only-pkey-more-aggressively arch/x86/include/asm/pkeys.h --- a/arch/x86/include/asm/pkeys.h~pkeys-abandon-exec-only-pkey-more-aggressively 2018-05-09 09:20:22.295698398 -0700 +++ b/arch/x86/include/asm/pkeys.h 2018-05-09 09:20:22.300698398 -0700 @@ -2,6 +2,8 @@ #ifndef _ASM_X86_PKEYS_H #define _ASM_X86_PKEYS_H
+#define ARCH_DEFAULT_PKEY 0 + #define arch_max_pkey() (boot_cpu_has(X86_FEATURE_OSPKE) ? 16 : 1)
extern int arch_set_user_pkey_access(struct task_struct *tsk, int pkey, @@ -15,7 +17,7 @@ extern int __execute_only_pkey(struct mm static inline int execute_only_pkey(struct mm_struct *mm) { if (!boot_cpu_has(X86_FEATURE_OSPKE)) - return 0; + return ARCH_DEFAULT_PKEY;
return __execute_only_pkey(mm); } @@ -56,6 +58,14 @@ bool mm_pkey_is_allocated(struct mm_stru return false; if (pkey >= arch_max_pkey()) return false; + /* + * The exec-only pkey is set in the allocation map, but + * is not available to any of the user interfaces like + * mprotect_pkey(). + */ + if (pkey == mm->context.execute_only_pkey) + return false; + return mm_pkey_allocation_map(mm) & (1U << pkey); }
diff -puN arch/x86/mm/pkeys.c~pkeys-abandon-exec-only-pkey-more-aggressively arch/x86/mm/pkeys.c --- a/arch/x86/mm/pkeys.c~pkeys-abandon-exec-only-pkey-more-aggressively 2018-05-09 09:20:22.297698398 -0700 +++ b/arch/x86/mm/pkeys.c 2018-05-09 09:20:22.301698398 -0700 @@ -94,26 +94,27 @@ int __arch_override_mprotect_pkey(struct */ if (pkey != -1) return pkey; - /* - * Look for a protection-key-drive execute-only mapping - * which is now being given permissions that are not - * execute-only. Move it back to the default pkey. - */ - if (vma_is_pkey_exec_only(vma) && - (prot & (PROT_READ|PROT_WRITE))) { - return 0; - } + /* * The mapping is execute-only. Go try to get the * execute-only protection key. If we fail to do that, * fall through as if we do not have execute-only - * support. + * support in this mm. */ if (prot == PROT_EXEC) { pkey = execute_only_pkey(vma->vm_mm); if (pkey > 0) return pkey; + } else if (vma_is_pkey_exec_only(vma)) { + /* + * Protections are *not* PROT_EXEC, but the mapping + * is using the exec-only pkey. This mapping was + * PROT_EXEC and will no longer be. Move back to + * the default pkey. + */ + return ARCH_DEFAULT_PKEY; } + /* * This is a vanilla, non-pkey mprotect (or we failed to * setup execute-only), inherit the pkey from the VMA we _