Re: [PATCH] f2fs: allocate buffer for decrypting filename to avoid panic

[Yunlong Song]

Hi, Eric,
    Thanks for your review, I have removed the symlink part and use
kmemdup instead. As for directory pages restricted to lowmem, I am
not sure whether it is proper for f2fs, since the directory structures
of f2fs are different from ext4. So I just keep its old kmap.

On 2018/2/25 2:32, Eric Biggers wrote:

Hi Yunlong,

On Sat, Feb 24, 2018 at 05:14:58PM +0800, Yunlong Song wrote:

In some platforms (such as arm), high memory is used, then the
decrypting filename will cause panic, the reason see commit
569cf1876a32e574ba8a7fb825cd91bafd003882 ("f2fs crypto: allocate buffer
for decrypting filename"):

  We got dentry pages from high_mem, and its address space directly goes into 
the
  decryption path via f2fs_fname_disk_to_usr.
  But, sg_init_one assumes the address is not from high_mem, so we can get this
  panic since it doesn't call kmap_high but kunmap_high is triggered at the end.

  kernel BUG at ../../../../../../kernel/mm/highmem.c:290!
  Internal error: Oops - BUG: 0 [#1] PREEMPT SMP ARM
  ...
   (kunmap_high+0xb0/0xb8) from [<c0114534>] (__kunmap_atomic+0xa0/0xa4)
   (__kunmap_atomic+0xa0/0xa4) from [<c035f028>] 
(blkcipher_walk_done+0x128/0x1ec)
   (blkcipher_walk_done+0x128/0x1ec) from [<c0366c24>] 
(crypto_cbc_decrypt+0xc0/0x170)
   (crypto_cbc_decrypt+0xc0/0x170) from [<c0367148>] 
(crypto_cts_decrypt+0xc0/0x114)
   (crypto_cts_decrypt+0xc0/0x114) from [<c035ea98>] (async_decrypt+0x40/0x48)
   (async_decrypt+0x40/0x48) from [<c032ca34>] 
(f2fs_fname_disk_to_usr+0x124/0x304)
   (f2fs_fname_disk_to_usr+0x124/0x304) from [<c03056fc>] 
(f2fs_fill_dentries+0xac/0x188)
   (f2fs_fill_dentries+0xac/0x188) from [<c03059c8>] (f2fs_readdir+0x1f0/0x300)
   (f2fs_readdir+0x1f0/0x300) from [<c0218054>] (vfs_readdir+0x90/0xb4)
   (vfs_readdir+0x90/0xb4) from [<c0218418>] (SyS_getdents64+0x64/0xcc)
   (SyS_getdents64+0x64/0xcc) from [<c0105ba0>] (ret_fast_syscall+0x0/0x30)

Howerver, later patches:
commit 922ec355f86365388203672119b5bca346a45085 ("f2fs crypto: avoid
unneeded memory allocation when {en/de}crypting symlink")
commit e06f86e61d7a67fe6e826010f57aa39c674f4b1b ("f2fs crypto: avoid
unneeded memory allocation in ->readdir")

reverts the codes, which causes panic again in arm, so let's add the old
patch again.

Signed-off-by: Yunlong Song <yunlong.s...@huawei.com>
---
  fs/f2fs/dir.c   |  7 +++++++
  fs/f2fs/namei.c | 10 +++++++++-
  2 files changed, 16 insertions(+), 1 deletion(-)

diff --git a/fs/f2fs/dir.c b/fs/f2fs/dir.c
index f00b5ed..c0cf3e7b 100644
--- a/fs/f2fs/dir.c
+++ b/fs/f2fs/dir.c
@@ -825,9 +825,16 @@ int f2fs_fill_dentries(struct dir_context *ctx, struct 
f2fs_dentry_ptr *d,
                        int save_len = fstr->len;
                        int err;

+ de_name.name = kmalloc(de_name.len, GFP_NOFS);

+                       if (!de_name.name)
+                               return -ENOMEM;
+
+                       memcpy(de_name.name, d->filename[bit_pos], de_name.len);
+
                        err = fscrypt_fname_disk_to_usr(d->inode,
                                                (u32)de->hash_code, 0,
                                                &de_name, fstr);
+                       kfree(de_name.name);
                        if (err)
                                return err;

diff --git a/fs/f2fs/namei.c b/fs/f2fs/namei.c

index c4c94c7..2cb70c1 100644
--- a/fs/f2fs/namei.c
+++ b/fs/f2fs/namei.c
@@ -1170,8 +1170,13 @@ static const char *f2fs_encrypted_get_link(struct dentry 
*dentry,

/* Symlink is encrypted */

        sd = (struct fscrypt_symlink_data *)caddr;
-       cstr.name = sd->encrypted_path;
        cstr.len = le16_to_cpu(sd->len);
+       cstr.name = kmalloc(cstr.len, GFP_NOFS);
+       if (!cstr.name) {
+               res = -ENOMEM;
+               goto errout;
+       }
+       memcpy(cstr.name, sd->encrypted_path, cstr.len);

/* this is broken symlink case */

        if (unlikely(cstr.len == 0)) {
@@ -1198,6 +1203,8 @@ static const char *f2fs_encrypted_get_link(struct dentry 
*dentry,
                goto errout;
        }

+ kfree(cstr.name);

+
        paddr = pstr.name;

/* Null-terminate the name */

@@ -1207,6 +1214,7 @@ static const char *f2fs_encrypted_get_link(struct dentry 
*dentry,
        set_delayed_call(done, kfree_link, paddr);
        return paddr;
  errout:
+       kfree(cstr.name);
        fscrypt_fname_free_buffer(&pstr);
        put_page(cpage);
        return ERR_PTR(res);
--
1.8.5.2

The pagecache for symlinks in f2fs already uses lowmem only, so the change to
->get_link() isn't needed.  Also that part of the patch doesn't apply to
upstream.

For directories we may need your fix.  Note: kmalloc + memcpy should be replaced
with kmemdup.  But alternatively, directory pages could be restricted to lowmem
which would match ext4.

Eric

.

--
Thanks,
Yunlong Song