From bf331efc8ea4102cbac016511d131a68efae7bcb Mon Sep 17 00:00:00 2001 From: Eric Biggers Date: Wed, 15 Jul 2020 23:05:53 -0700 Subject: [PATCH] /dev/mem: Add missing memory barriers for devmem_inode commit b34e7e298d7a5ed76b3aa327c240c29f1ef6dd22 upstream. WRITE_ONCE() isn't the correct way to publish a pointer to a data structure, since it doesn't include a write memory barrier. Therefore other tasks may see that the pointer has been set but not see that the pointed-to memory has finished being initialized yet. Instead a primitive with "release" semantics is needed. Use smp_store_release() for this. The use of READ_ONCE() on the read side is still potentially correct if there's no control dependency, i.e. if all memory being "published" is transitively reachable via the pointer itself. But this pairing is somewhat confusing and error-prone. So just upgrade the read side to smp_load_acquire() so that it clearly pairs with smp_store_release(). Cc: Arnd Bergmann Cc: Ingo Molnar Cc: Kees Cook Cc: Matthew Wilcox Cc: Russell King Cc: Andrew Morton Fixes: 3234ac664a87 ("/dev/mem: Revoke mappings when a driver claims the region") Signed-off-by: Eric Biggers Cc: stable Acked-by: Dan Williams Link: https://lore.kernel.org/r/20200716060553.24618-1-ebiggers@kernel.org Signed-off-by: Greg Kroah-Hartman --- drivers/char/mem.c | 10 +++++++--- 1 file changed, 7 insertions(+), 3 deletions(-) diff --git a/drivers/char/mem.c b/drivers/char/mem.c index 31cae88a730b..6b56bff9b68c 100644 --- a/drivers/char/mem.c +++ b/drivers/char/mem.c @@ -814,7 +814,8 @@ static struct inode *devmem_inode; #ifdef CONFIG_IO_STRICT_DEVMEM void revoke_devmem(struct resource *res) { - struct inode *inode = READ_ONCE(devmem_inode); + /* pairs with smp_store_release() in devmem_init_inode() */ + struct inode *inode = smp_load_acquire(&devmem_inode); /* * Check that the initialization has completed. Losing the race @@ -1028,8 +1029,11 @@ static int devmem_init_inode(void) return rc; } - /* publish /dev/mem initialized */ - WRITE_ONCE(devmem_inode, inode); + /* + * Publish /dev/mem initialized. + * Pairs with smp_load_acquire() in revoke_devmem(). + */ + smp_store_release(&devmem_inode, inode); return 0; }