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)
[<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@xxxxxxxxxxx>
Fixes: daae28debcb0 ("hwrng: core - move add_early_randomness() out of rng_mutex")
Tested-by: Marek Szyprowski <m.szyprowski@xxxxxxxxxxx>
Signed-off-by: Laurent Vivier <lvivier@xxxxxxxxxx>
---
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 @@ int hwrng_register(struct hwrng *rng)
* 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;
}
--
2.21.0
