> On Wed, 2023-10-11 at 16:43 +0800, 黄思聪 wrote:
> > Pointer dereference error may occur in "alloc_init_deleg" function.
> > 
> > The "alloc_init_deleg" function located in "fs/nfsd/nfs4state.c" may occur a pointer dereference error when it calls the function "nfs4_alloc_stid" located in the same kernel file. The "nfs4_alloc_stid" function will call the "kmem_cache_zalloc" function to allocate enough memory for storing the "stid" variable. If there are significant memory fragmentation issues, insufficient free memory blocks, or internal errors in the allocation function, the "kmem_cache_zalloc" function will return NULL. Then the "nfs4_alloc_stid" function will return NULL to the "alloc_init_deleg" function. Finally, the "alloc_init_deleg" function will execute the following instructions.
> > dp = delegstateid(nfs4_alloc_stid(clp, deleg_slab, nfs4_free_deleg));  
> > if (dp == NULL)  
> >         goto out_dec;
> > dp->dl_stid.sc_stateid.si_generation = 1;
> > 
> > The "delegstateid" function is defined as below:
> > static inline struct nfs4_delegation *delegstateid(struct nfs4_stid *s)  
> > {  
> >         return container_of(s, struct nfs4_delegation, dl_stid);  
> > }
> > 
> > When the parameter "struct nfs4_stid *s" is NULL, the function will return a strange value which is a negative number. The value will be interpreted as a very large number. Then the variable "dp" in the "alloc_init_deleg" function will get the value, and it will pass the following "if" conditional statements. In the last, the variable "dp" will be dereferenced, and it will cause an error.
> > 
> > My experimental kernel version is "LINUX 6.1", and this problem exists in all the version from "LINUX v3.2-rc1" to "LINUX v6.6-rc5".
> 
> 
> (I don't think there are security implications here, so I'm cc'ing the
> mailing list and making this public.)
> 
> Well spotted! Ordinarily you'd be correct, but dl_stid is the first
> field in the struct, so the container_of will just return the same
> value that you pass in.
> 
> Still, this is not something we ought to rely on going forward. Would
> you care to make a patch to clean this up and make that a bit less
> subtle?
> 
> Thanks!
> -- 
> Jeff Layton [off-list ref]


Thank you for your feedback! Indeed, you are correct! Next time I will check it twice before reporting a problem.

My patch is below:

Modify the conditional statement for null pointer check in the function
'alloc_init_deleg' to make this function more robust and clear. Otherwise,
this function may have potential pointer dereference problem in the future,
when modifying or expanding the nfs4_delegation structure.

Signed-off-by: Sicong Huang <redacted>
---
 fs/nfsd/nfs4state.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/fs/nfsd/nfs4state.c b/fs/nfsd/nfs4state.c
index b1118050ff52..516b8bd6cb53 100644
--- a/fs/nfsd/nfs4state.c
+++ b/fs/nfsd/nfs4state.c

@@ -1160,6 +1160,7 @@ alloc_init_deleg(struct nfs4_client *clp, struct nfs4_file *fp,
 		 struct nfs4_clnt_odstate *odstate, u32 dl_type)
 {
 	struct nfs4_delegation *dp;
+	struct nfs4_stid *stid;
 	long n;
 
 	dprintk("NFSD alloc_init_deleg\n");

@@ -1168,9 +1169,10 @@ alloc_init_deleg(struct nfs4_client *clp, struct nfs4_file *fp,
 		goto out_dec;
 	if (delegation_blocked(&fp->fi_fhandle))
 		goto out_dec;
-	dp = delegstateid(nfs4_alloc_stid(clp, deleg_slab, nfs4_free_deleg));
-	if (dp == NULL)
+	stid = nfs4_alloc_stid(clp, deleg_slab, nfs4_free_deleg);
+	if (stid == NULL)
 		goto out_dec;
+	dp = delegstateid(stid);
 
 	/*
 	 * delegation seqid's are never incremented.  The 4.1 special

-- 
2.34.1

Best Regards,
Sicong Huang
</huangsicong@iie.ac.cn></jlayton@kernel.org>