[PATCH] fs: pstore: fix double-free on ramoops_init_przs

[PATCH] fs: pstore: fix double-free on ramoops_init_przs

[Cengiz Can]

According to Coverity scanner (CID 1457526) kfree on ram.c:591 frees
label which has already been freed.

Here's the flow as I have understood (this is my first time reading
pstore's files):

Whenever `persistent_ram_new` fails, it implicitly calls
`persistent_ram_free(prz)` which already does `kfree(prz->label)` and a
`kfree(prz)` consequently.

Removed `kfree(label)` to prevent double-free.

Signed-off-by: Cengiz Can <cengiz@kernel.wtf>
---
 fs/pstore/ram.c | 1 -
 1 file changed, 1 deletion(-)

diff --git a/fs/pstore/ram.c b/fs/pstore/ram.c
index 487ee39b4..e196aa08f 100644
--- a/fs/pstore/ram.c
+++ b/fs/pstore/ram.c
@@ -588,7 +588,6 @@ static int ramoops_init_przs(const char *name,
 			dev_err(dev, "failed to request %s mem region (0x%zx@0x%llx): %d\n",
 				name, record_size,
 				(unsigned long long)*paddr, err);
-			kfree(label);
 
 			while (i > 0) {
 				i--;
-- 
2.24.1

Re: [PATCH] fs: pstore: fix double-free on ramoops_init_przs

[Kees Cook]

On Tue, Jan 07, 2020 at 02:04:46PM +0300, Cengiz Can wrote:
> According to Coverity scanner (CID 1457526) kfree on ram.c:591 frees
> label which has already been freed.
> 
> Here's the flow as I have understood (this is my first time reading
> pstore's files):
> 
> Whenever `persistent_ram_new` fails, it implicitly calls
> `persistent_ram_free(prz)` which already does `kfree(prz->label)` and a
> `kfree(prz)` consequently.
> 
> Removed `kfree(label)` to prevent double-free.

I think this is a false positive (have you actually hit the
double-free?). The logic in this area is:

label = kmalloc(...)
prz[i] = persistent_ram_new(..., label, ...)
if (IS_ERR(prz[i])) {
    kfree(label)
    while (i > 0) {
        i--;
        persistent_ram_free(prz[i]);
    }
}

nothing was freeing the label on the failed prz, but all the other prz
labels were free (i.e. there is a "i--" that skips the failed prz
alloc).

-Kees

> 
> Signed-off-by: Cengiz Can <cengiz@kernel.wtf>
> ---
>  fs/pstore/ram.c | 1 -
>  1 file changed, 1 deletion(-)
> 
> diff --git a/fs/pstore/ram.c b/fs/pstore/ram.c
> index 487ee39b4..e196aa08f 100644
> --- a/fs/pstore/ram.c
> +++ b/fs/pstore/ram.c
> @@ -588,7 +588,6 @@ static int ramoops_init_przs(const char *name,
>  			dev_err(dev, "failed to request %s mem region (0x%zx@0x%llx): %d\n",
>  				name, record_size,
>  				(unsigned long long)*paddr, err);
> -			kfree(label);
>  
>  			while (i > 0) {
>  				i--;
> -- 
> 2.24.1
> 

-- 
Kees Cook

Re: [PATCH] fs: pstore: fix double-free on ramoops_init_przs

[Cengiz Can]

Hello Kees!

It's a pleasure to hear from you!

On 2020-01-07 21:05, Kees Cook wrote:
> 
> I think this is a false positive (have you actually hit the
> double-free?). The logic in this area is:

No I did not actually hit the double-free. I'm just following
the indicators from static analyzer.

> nothing was freeing the label on the failed prz, but all the other prz
> labels were free (i.e. there is a "i--" that skips the failed prz
> alloc).

I have noticed that. Thanks for clearing it up though.

The `kfree` I was referring to is in `err:` label of function
`persistent_ram_new` in `ram_core.c#595` of `for-next/pstore` tree:

https://git.kernel.org/pub/scm/linux/kernel/git/kees/linux.git/tree/fs/pstore/ram_core.c?h=for-next/pstore#n595

Here are the relevant bits:

```
struct persistent_ram_zone *persistent_ram_new(phys_addr_t start, size_t 
size,
			u32 sig, struct persistent_ram_ecc_info *ecc_info,
			unsigned int memtype, u32 flags, char *label)
{
	/* ... */
	/* ... */
	/* ... */
	return prz;
err:
	persistent_ram_free(prz); /* <----- */
	return ERR_PTR(ret);
}
```

So, to my understanding, if our `persistent_ram_new` call in `ram.c#583`
fails, it already does clean up the `label` pointer in itself and 
returns
an ERR_PTR back to us and our skipping logic does its job.

I might be missing something but it seems so.

Thank you for looking into this.

-- 
Cengiz Can
@cengiz_io

Re: [PATCH] fs: pstore: fix double-free on ramoops_init_przs

[Kees Cook]

On Tue, Jan 07, 2020 at 10:40:58PM +0300, Cengiz Can wrote:
> Hello Kees!
> 
> It's a pleasure to hear from you!
> 
> On 2020-01-07 21:05, Kees Cook wrote:
> > 
> > I think this is a false positive (have you actually hit the
> > double-free?). The logic in this area is:
> 
> No I did not actually hit the double-free. I'm just following
> the indicators from static analyzer.
> 
> > nothing was freeing the label on the failed prz, but all the other prz
> > labels were free (i.e. there is a "i--" that skips the failed prz
> > alloc).
> 
> I have noticed that. Thanks for clearing it up though.
> 
> The `kfree` I was referring to is in `err:` label of function
> `persistent_ram_new` in `ram_core.c#595` of `for-next/pstore` tree:
> 
> https://git.kernel.org/pub/scm/linux/kernel/git/kees/linux.git/tree/fs/pstore/ram_core.c?h=for-next/pstore#n595
> 
> Here are the relevant bits:
> 
> ```
> struct persistent_ram_zone *persistent_ram_new(phys_addr_t start, size_t
> size,
> 			u32 sig, struct persistent_ram_ecc_info *ecc_info,
> 			unsigned int memtype, u32 flags, char *label)
> {
> 	/* ... */
> 	/* ... */
> 	/* ... */
> 	return prz;
> err:
> 	persistent_ram_free(prz); /* <----- */
> 	return ERR_PTR(ret);
> }
> ```
> 
> So, to my understanding, if our `persistent_ram_new` call in `ram.c#583`
> fails, it already does clean up the `label` pointer in itself and returns
> an ERR_PTR back to us and our skipping logic does its job.
> 
> I might be missing something but it seems so.
> 
> Thank you for looking into this.

Ah-ha! Yes, I see it now. We have multiple paths to the err: label, and
I was focused on the kzalloc() failure path. I will get this fixed
better. Thanks!

-- 
Kees Cook