[PATCH] powerpc: Initialize local variable fdt to NULL in elf64_load()

[Michael Ellerman]

Lakshmi Ramasubramanian <nramas at linux.microsoft.com> writes:
> On 4/16/21 2:05 AM, Michael Ellerman wrote:
>
>> Daniel Axtens <dja at axtens.net> writes:
>>>> On 4/15/21 12:14 PM, Lakshmi Ramasubramanian wrote:
>>>>
>>>> Sorry - missed copying device-tree and powerpc mailing lists.
>>>>
>>>>> There are a few "goto out;" statements before the local variable "fdt"
>>>>> is initialized through the call to of_kexec_alloc_and_setup_fdt() in
>>>>> elf64_load(). This will result in an uninitialized "fdt" being passed
>>>>> to kvfree() in this function if there is an error before the call to
>>>>> of_kexec_alloc_and_setup_fdt().
>>>>>
>>>>> Initialize the local variable "fdt" to NULL.
>>>>>
>>> I'm a huge fan of initialising local variables! But I'm struggling to
>>> find the code path that will lead to an uninit fdt being returned...
>>>
>>> The out label reads in part:
>>>
>>> 	/* Make kimage_file_post_load_cleanup free the fdt buffer for us. */
>>> 	return ret ? ERR_PTR(ret) : fdt;
>>>
>>> As far as I can tell, any time we get a non-zero ret, we're going to
>>> return an error pointer rather than the uninitialised value...
>
> As Dan pointed out, the new code is in linux-next.
>
> I have copied the new one below - the function doesn't return fdt, but 
> instead sets it in the arch specific field (please see the link to the 
> updated elf_64.c below).
>
> https://git.kernel.org/pub/scm/linux/kernel/git/robh/linux.git/tree/arch/powerpc/kexec/elf_64.c?h=for-next
>
>>>
>>> (btw, it does look like we might leak fdt if we have an error after we
>>> successfully kmalloc it.)
>>>
>>> Am I missing something? Can you link to the report for the kernel test
>>> robot or from Dan?
>
> /*
>           * Once FDT buffer has been successfully passed to 
> kexec_add_buffer(),
>           * the FDT buffer address is saved in image->arch.fdt. In that 
> case,
>           * the memory cannot be freed here in case of any other error.
>           */
>          if (ret && !image->arch.fdt)
>                  kvfree(fdt);
>
>          return ret ? ERR_PTR(ret) : NULL;
>
> In case of an error, the memory allocated for fdt is freed unless it has 
> already been passed to kexec_add_buffer().

It feels like the root of the problem is that the kvfree of fdt is in
the wrong place. It's only allocated later in the function, so the error
path should reflect that. Something like the patch below.

cheers


diff --git a/arch/powerpc/kexec/elf_64.c b/arch/powerpc/kexec/elf_64.c
index 5a569bb51349..02662e72c53d 100644
--- a/arch/powerpc/kexec/elf_64.c
+++ b/arch/powerpc/kexec/elf_64.c
@@ -114,7 +114,7 @@ static void *elf64_load(struct kimage *image, char *kernel_buf,
 	ret = setup_new_fdt_ppc64(image, fdt, initrd_load_addr,
 				  initrd_len, cmdline);
 	if (ret)
-		goto out;
+		goto out_free_fdt;
 
 	fdt_pack(fdt);
 
@@ -125,7 +125,7 @@ static void *elf64_load(struct kimage *image, char *kernel_buf,
 	kbuf.mem = KEXEC_BUF_MEM_UNKNOWN;
 	ret = kexec_add_buffer(&kbuf);
 	if (ret)
-		goto out;
+		goto out_free_fdt;
 
 	/* FDT will be freed in arch_kimage_file_post_load_cleanup */
 	image->arch.fdt = fdt;
@@ -140,18 +140,14 @@ static void *elf64_load(struct kimage *image, char *kernel_buf,
 	if (ret)
 		pr_err("Error setting up the purgatory.\n");
 
+	goto out;
+
+out_free_fdt:
+	kvfree(fdt);
 out:
 	kfree(modified_cmdline);
 	kexec_free_elf_info(&elf_info);
 
-	/*
-	 * Once FDT buffer has been successfully passed to kexec_add_buffer(),
-	 * the FDT buffer address is saved in image->arch.fdt. In that case,
-	 * the memory cannot be freed here in case of any other error.
-	 */
-	if (ret && !image->arch.fdt)
-		kvfree(fdt);
-
 	return ret ? ERR_PTR(ret) : NULL;
 }