Commit 28443671 authored Oct 14, 2019 by Laurent Vivier Committed by Herbert Xu Oct 23, 2019

hwrng: core - Fix use-after-free warning in hwrng_register()



Commit daae28de 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)
[<c01124c8>] (unwind_backtrace) from [<c010dfb8>] (show_stack+0x10/0x14)
[<c010dfb8>] (show_stack) from [<c0ae86d8>] (dump_stack+0xa8/0xd4)
[<c0ae86d8>] (dump_stack) from [<c0127428>] (__warn+0xf4/0x10c)
[<c0127428>] (__warn) from [<c01274b4>] (warn_slowpath_fmt+0x74/0xb8)
[<c01274b4>] (warn_slowpath_fmt) from [<c054729c>] (hwrng_register+0x13c/0x1b4)
[<c054729c>] (hwrng_register) from [<c0547e54>] (tpm_chip_register+0xc4/0x274)
...

Reported-by: Marek Szyprowski <m.szyprowski@samsung.com>
Fixes: daae28de ("hwrng: core - move add_early_randomness() out of rng_mutex")
Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Laurent Vivier <lvivier@redhat.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>

parent 4ba80298

drivers/char/hw_random/core.c

+16 −17

Original line number	Diff line number	Diff line
		@@ -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;
		}

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