[RFC PATCH 4/6] pkeys: Lift pkey hardware check for pkey_alloc()

[ira . weiny]

From: Ira Weiny <ira.weiny@xxxxxxxxx>

pkey_alloc() is documented to return ENOSPC when the hardware does not
support pkeys.  On x86, pkey_alloc() incorrectly returns EINVAL.

This is because mm_pkey_alloc() does not check for pkey support before
returning a key.  Therefore, if the keys are not exhausted pkey_alloc()
continues on to call arch_set_user_pkey_access().  Unfortunately, when
arch_set_user_pkey_access() detects the failed support it overwrites the
ENOSPC return value with EINVAL.

Ensure consistent behavior across architectures by lifting this check to
the core mm code.

Remove a couple of 'we' references in code comments as well.

Cc: ahaas@xxxxxxxxxxxx
Cc: clemensb@xxxxxxxxxxxx
Cc: gdeepti@xxxxxxxxxxxx
Cc: jkummerow@xxxxxxxxxxxx
Cc: manoskouk@xxxxxxxxxxxx
Cc: thibaudm@xxxxxxxxxxxx
Cc: Florian Weimer <fweimer@xxxxxxxxxx>
Cc: Sohil Mehta <sohil.mehta@xxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
Cc: Aneesh Kumar K.V <aneesh.kumar@xxxxxxxxxxxxx>
Cc: linux-api@xxxxxxxxxxxxxxx
Fixes: e8c24d3a23a4 ("x86/pkeys: Allocation/free syscalls")
Signed-off-by: Ira Weiny <ira.weiny@xxxxxxxxx>

---
Thanks to Sohil for pointing out that the commit message could be more
clear WRT how EINVAL is returned incorrectly.
---
 arch/powerpc/include/asm/pkeys.h | 8 +++-----
 mm/mprotect.c                    | 3 +++
 2 files changed, 6 insertions(+), 5 deletions(-)

diff --git a/arch/powerpc/include/asm/pkeys.h b/arch/powerpc/include/asm/pkeys.h
index 59a2c7dbc78f..2c8351248793 100644
--- a/arch/powerpc/include/asm/pkeys.h
+++ b/arch/powerpc/include/asm/pkeys.h
@@ -85,18 +85,16 @@ static inline bool mm_pkey_is_allocated(struct mm_struct *mm, int pkey)
 static inline int mm_pkey_alloc(struct mm_struct *mm)
 {
 	/*
-	 * Note: this is the one and only place we make sure that the pkey is
+	 * Note: this is the one and only place to make sure that the pkey is
 	 * valid as far as the hardware is concerned. The rest of the kernel
 	 * trusts that only good, valid pkeys come out of here.
 	 */
 	u32 all_pkeys_mask = (u32)(~(0x0));
 	int ret;
 
-	if (!mmu_has_feature(MMU_FTR_PKEY))
-		return -1;
 	/*
-	 * Are we out of pkeys? We must handle this specially because ffz()
-	 * behavior is undefined if there are no zeros.
+	 * Out of pkeys?  Handle this specially because ffz() behavior is
+	 * undefined if there are no zeros.
 	 */
 	if (mm_pkey_allocation_map(mm) == all_pkeys_mask)
 		return -1;
diff --git a/mm/mprotect.c b/mm/mprotect.c
index ba5592655ee3..56d35de33725 100644
--- a/mm/mprotect.c
+++ b/mm/mprotect.c
@@ -773,6 +773,9 @@ SYSCALL_DEFINE2(pkey_alloc, unsigned long, flags, unsigned long, init_val)
 	int pkey;
 	int ret;
 
+	if (!arch_pkeys_enabled())
+		return -ENOSPC;
+
 	/* No flags supported yet. */
 	if (flags)
 		return -EINVAL;
-- 
2.35.1