Re: [PATCH] hwrng: core - Fix use-after-free warning in hwrng_register()

[Herbert Xu]

On Mon, Oct 14, 2019 at 01:46:32PM +0200, Laurent Vivier wrote:
> 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(-)

Patch applied.  Thanks.
-- 
Email: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>
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