linux-kernel - [PATCH RFC v1] clk: Fix potential NULL dereference in clk_fetch_parent

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Date:   Fri, 16 Aug 2019 00:31:55 +0200
From:   Martin Blumenstingl <martin.blumenstingl@...glemail.com>
To:     linux-clk@...r.kernel.org, sboyd@...nel.org
Cc:     linux-kernel@...r.kernel.org, mturquette@...libre.com,
        Martin Blumenstingl <martin.blumenstingl@...glemail.com>
Subject: [PATCH RFC v1] clk: Fix potential NULL dereference in clk_fetch_parent_index()

Don't compare the parent clock name with a NULL name in the
clk_parent_map. This prevents a kernel crash when passing NULL
core->parents[i].name to strcmp().

An example which triggered this is a mux clock with four parents when
each of them is referenced in the clock driver using
clk_parent_data.fw_name and then calling clk_set_parent(clk, 3rd_parent)
on this mux.
In this case the first parent is also the HW default so
core->parents[i].hw is populated when the clock is registered. Calling
clk_set_parent(clk, 3rd_parent) will then go through all parents and
skip the first parent because it's hw pointer doesn't match. For the
second parent no hw pointer is cached yet and clk_core_get(core, 1)
returns a non-matching pointer (which is correct because we are comparing
the second with the third parent). Comparing the result of
clk_core_get(core, 2) with the requested parent gives a match. However
we don't reach this point because right after the clk_core_get(core, 1)
mismatch the old code tried to !strcmp(parent->name, NULL) (where the
second argument is actually core->parents[i].name, but that was never
populated by the clock driver).

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@...glemail.com>
---
I have seen the original crash when I was testing an MMC driver which
is not upstream yet on v5.3-rc4. I'm not sure whether this fix is
"correct" (it fixes the crash for me) or where to point the Fixes tag
to, it may be one of:
- fc0c209c147f ("clk: Allow parents to be specified without string names")
- 1a079560b145 ("clk: Cache core in clk_fetch_parent_index() without names")

This is meant to be applied on top of v5.3-rc4.

 drivers/clk/clk.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/clk/clk.c b/drivers/clk/clk.c
index c0990703ce54..567a044a368b 100644
--- a/drivers/clk/clk.c
+++ b/drivers/clk/clk.c
@@ -1632,7 +1632,8 @@ static int clk_fetch_parent_index(struct clk_core *core,
 			break;

 		/* Fallback to comparing globally unique names */
-		if (!strcmp(parent->name, core->parents[i].name))
+		if (core->parents[i].name &&
+		    !strcmp(parent->name, core->parents[i].name))
 			break;
 	}

-- 
2.22.1