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From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     linux-kernel@vger.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        stable@vger.kernel.org, Gary R Hook <gary.hook@amd.com>,
        PrasannaKumar Muralidharan <prasannatsmkumar@gmail.com>,
        Herbert Xu <herbert@gondor.apana.org.au>,
        Sasha Levin <alexander.levin@microsoft.com>
Subject: [PATCH 4.15 75/84] hwrng: core - Clean up RNG list when last hwrng is unregistered
Date:   Fri, 23 Mar 2018 10:54:29 +0100
Message-Id: <20180323095423.243279414@linuxfoundation.org>
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4.15-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Gary R Hook <gary.hook@amd.com>


[ Upstream commit 0e4b52942b1c76f89e0dcb829f72e123d0678f54 ]

Commit 142a27f0a731 added support for a "best" RNG, and in doing so
introduced a hang from rmmod/modprobe -r when the last RNG on the list
was unloaded.

When the hwrng list is depleted, return the global variables to their
original state and decrement all references to the object.

Fixes: 142a27f0a731 ("hwrng: core - Reset user selected rng by writing "" to rng_current")
Signed-off-by: Gary R Hook <gary.hook@amd.com>
Reviewed-by: PrasannaKumar Muralidharan <prasannatsmkumar@gmail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 drivers/char/hw_random/core.c |    4 ++++
 1 file changed, 4 insertions(+)

--- a/drivers/char/hw_random/core.c
+++ b/drivers/char/hw_random/core.c
@@ -306,6 +306,10 @@ static int enable_best_rng(void)
 		ret = ((new_rng == current_rng) ? 0 : set_current_rng(new_rng));
 		if (!ret)
 			cur_rng_set_by_user = 0;
+	} else {
+		drop_current_rng();
+		cur_rng_set_by_user = 0;
+		ret = 0;
 	}
 
 	return ret;