Re: [PATCH V1] nvmem: core: fix memory abort in cleanup path

From: Bitan Biswas
Date: Fri Jan 03 2020 - 07:50:19 EST

Hi Thierry,

On 1/2/20 11:11 PM, Thierry Reding wrote:
On Thu, Jan 02, 2020 at 10:51:24AM -0800, Bitan Biswas wrote:

Hi Thierry,

On 1/2/20 4:44 AM, Thierry Reding wrote:
On Sat, Dec 28, 2019 at 08:02:42PM -0800, Bitan Biswas wrote:
nvmem_cell_info_to_nvmem_cell implementation has static
allocation of name. nvmem_add_cells_from_of() call may
return error and kfree name results in memory abort. Use
kasprintf() instead of assigning pointer and prevent kfree crash.

diff --git a/drivers/nvmem/core.c b/drivers/nvmem/core.c
index 9f1ee9c..0fc66e1 100644
--- a/drivers/nvmem/core.c
+++ b/drivers/nvmem/core.c
@@ -110,7 +110,7 @@ static int nvmem_cell_info_to_nvmem_cell(struct nvmem_device *nvmem,
cell->nvmem = nvmem;
cell->offset = info->offset;
cell->bytes = info->bytes;
- cell->name = info->name;
+ cell->name = kasprintf(GFP_KERNEL, "%s", info->name);

kstrdup() seems more appropriate here.
Thanks. I shall update the patch as suggested.

A slightly more efficient way to do this would be to use a combination
of kstrdup_const() and kfree_const(), which would allow read-only
strings to be replicated by simple assignment rather than duplication.
Note that in that case you'd need to carefully replace all kfree() calls
on cell->name by a kfree_const() to ensure they do the right thing.
kfree(cell->name) is also called for allocations in function
nvmem_add_cells_from_of() through below call
kasprintf(GFP_KERNEL, "%pOFn", child);

My understanding is kfree_const may not work for above allocation.

kfree_const() checks the location that the pointer passed to it points
to. If it points to the kernel's .rodata section, it returns and only
calls kfree() otherwise. Similarily, kstrdup_const() returns its
argument if it points to the .rodata section and duplicates the string
otherwise. On the other hand, pointers returned by kasprintf() will
never point to the .rodata section, so kfree_const() will result in
kfree() getting called.

That said, the savings here are fairly minimal, so I don't feel very
strongly about this. Feel free to go with the kstrdup() variant.
Thanks for the explanation. I would test the implementation with the _const functions you suggested and send updated patch.