x86/pkeys: properly copy pkey state at fork()

From: Dave Hansen <dave.hansen@linux.intel.com>

Memory protection key behavior should be the same in a child as it was
in the parent before a fork.  But, there is a bug that resets the
state in the child at fork instead of preserving it.

Our creation of new mm's is a bit convoluted.  At fork(), the code

	1. memcpy() the parent mm to initialize child
	2. mm_init() to initalize some select stuff stuff
	3. dup_mmap() to create true copies that memcpy()
	   did not do right.

For pkeys, we need to preserve two bits of state across a fork:
'execute_only_pkey' and 'pkey_allocation_map'.  Those are preserved by
the memcpy(), which I thought did the right thing.  But, mm_init()
calls init_new_context(), which I thought was *only* for execve()-time
and overwrites 'execute_only_pkey' and 'pkey_allocation_map' with
"new" values.  But, alas, init_new_context() is used at execve() and

The result is that, after a fork(), the child's pkey state ends up
looking like it does after an execve(), which is totally wrong.  pkeys
that are already allocated can be allocated again, for instance.

To fix this, add code called by dup_mmap() to copy the pkey state from
parent to child explicitly.  Also add a comment above init_new_context()
to make it more clear to the next poor sod what this code is used for.

Fixes: e8c24d3a23a ("x86/pkeys: Allocation/free syscalls")
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: x86@kernel.org
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Will Deacon <will.deacon@arm.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Joerg Roedel <jroedel@suse.de>
Cc: stable@vger.kernel.org
1 file changed