Re: [PATCH RFC v1] clk: Fix potential NULL dereference in clk_fetch_parent_index()

[Stephen Boyd <sboyd@kernel.org>]

Quoting Martin Blumenstingl (2019-08-15 15:31:55)
> 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@googlemail.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.
> 

Ah ok. I thought that strcmp() would ignore NULL arguments, but
apparently not. I can apply this to clk-fixes.