From 28443671a468489fb147b31687b2157b6ba59c4a Mon Sep 17 00:00:00 2001 From: Laurent Vivier Date: Mon, 14 Oct 2019 13:46:32 +0200 Subject: hwrng: core - Fix use-after-free warning in hwrng_register() Commit daae28debcb0 has moved add_early_randomness() out of the rng_mutex and tries to protect the reference of the new rng device by incrementing the reference counter. But in hwrng_register(), the function can be called with a new device that is not set as the current_rng device and the reference has not been initialized. This patch fixes the problem by not using the reference counter when the device is not the current one: the reference counter is only meaningful in the case of the current rng device and a device is not used if it is not the current one (except in hwrng_register()) The problem has been reported by Marek Szyprowski on ARM 32bit Exynos5420-based Chromebook Peach-Pit board: WARNING: CPU: 3 PID: 1 at lib/refcount.c:156 hwrng_register+0x13c/0x1b4 refcount_t: increment on 0; use-after-free. Modules linked in: CPU: 3 PID: 1 Comm: swapper/0 Not tainted 5.4.0-rc1-00061-gdaae28debcb0 Hardware name: SAMSUNG EXYNOS (Flattened Device Tree) [] (unwind_backtrace) from [] (show_stack+0x10/0x14) [] (show_stack) from [] (dump_stack+0xa8/0xd4) [] (dump_stack) from [] (__warn+0xf4/0x10c) [] (__warn) from [] (warn_slowpath_fmt+0x74/0xb8) [] (warn_slowpath_fmt) from [] (hwrng_register+0x13c/0x1b4) [] (hwrng_register) from [] (tpm_chip_register+0xc4/0x274) ... Reported-by: Marek Szyprowski Fixes: daae28debcb0 ("hwrng: core - move add_early_randomness() out of rng_mutex") Tested-by: Marek Szyprowski Signed-off-by: Laurent Vivier Signed-off-by: Herbert Xu --- drivers/char/hw_random/core.c | 33 ++++++++++++++++----------------- 1 file changed, 16 insertions(+), 17 deletions(-) diff --git a/drivers/char/hw_random/core.c b/drivers/char/hw_random/core.c index 29f50c045c92..d85c6e18a2d2 100644 --- a/drivers/char/hw_random/core.c +++ b/drivers/char/hw_random/core.c @@ -471,17 +471,15 @@ static void start_khwrngd(void) int hwrng_register(struct hwrng *rng) { int err = -EINVAL; - struct hwrng *old_rng, *new_rng, *tmp; + struct hwrng *tmp; struct list_head *rng_list_ptr; + bool is_new_current = false; if (!rng->name || (!rng->data_read && !rng->read)) goto out; mutex_lock(&rng_mutex); - old_rng = current_rng; - new_rng = NULL; - /* Must not register two RNGs with the same name. */ err = -EEXIST; list_for_each_entry(tmp, &rng_list, list) { @@ -500,9 +498,8 @@ int hwrng_register(struct hwrng *rng) } list_add_tail(&rng->list, rng_list_ptr); - err = 0; - if (!old_rng || - (!cur_rng_set_by_user && rng->quality > old_rng->quality)) { + if (!current_rng || + (!cur_rng_set_by_user && rng->quality > current_rng->quality)) { /* * Set new rng as current as the new rng source * provides better entropy quality and was not @@ -511,15 +508,14 @@ int hwrng_register(struct hwrng *rng) err = set_current_rng(rng); if (err) goto out_unlock; + /* to use current_rng in add_early_randomness() we need + * to take a ref + */ + is_new_current = true; + kref_get(&rng->ref); } - - new_rng = rng; - kref_get(&new_rng->ref); -out_unlock: mutex_unlock(&rng_mutex); - - if (new_rng) { - if (new_rng != old_rng || !rng->init) { + if (is_new_current || !rng->init) { /* * Use a new device's input to add some randomness to * the system. If this rng device isn't going to be @@ -527,10 +523,13 @@ out_unlock: * called yet by set_current_rng(); so only use the * randomness from devices that don't need an init callback */ - add_early_randomness(new_rng); - } - put_rng(new_rng); + add_early_randomness(rng); } + if (is_new_current) + put_rng(rng); + return 0; +out_unlock: + mutex_unlock(&rng_mutex); out: return err; } -- cgit v1.2.3