efi: READ_ONCE rng seed size before munmap

This function is consistent with using size instead of seed->size
(except for one place that this patch fixes), but it reads seed->size
without using READ_ONCE, which means the compiler might still do
something unwanted. So, this commit simply adds the READ_ONCE
wrapper.

Fixes: 636259880a7e ("efi: Add support for seeding the RNG from a UEFI ...")
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: linux-efi@vger.kernel.org
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20200217123354.21140-1-Jason@zx2c4.com
Link: https://lore.kernel.org/r/20200221084849.26878-5-ardb@kernel.org

diff --git a/drivers/firmware/efi/efi.c b/drivers/firmware/efi/efi.c
index 621220a..21ea99f 100644 (file)
--- a/drivers/firmware/efi/efi.c
+++ b/drivers/firmware/efi/efi.c
@@ -552,7 +552,7 @@ int __init efi_config_parse_tables(void *config_tables, int count, int sz,
 
                seed = early_memremap(efi.rng_seed, sizeof(*seed));
                if (seed != NULL) {
-                       size = seed->size;
+                       size = READ_ONCE(seed->size);
                        early_memunmap(seed, sizeof(*seed));
                } else {
                        pr_err("Could not map UEFI random seed!\n");
@@ -562,7 +562,7 @@ int __init efi_config_parse_tables(void *config_tables, int count, int sz,
                                              sizeof(*seed) + size);
                        if (seed != NULL) {
                                pr_notice("seeding entropy pool\n");
-                               add_bootloader_randomness(seed->bits, seed->size);
+                               add_bootloader_randomness(seed->bits, size);
                                early_memunmap(seed, sizeof(*seed) + size);
                        } else {
                                pr_err("Could not map UEFI random seed!\n");