Bug 213083

In "fs/udf/namei.c" there is a possible NULL pointer dereference bug in udf_symlink:882 function.

This is the relevant piece of code:
> 936         epos.bh = udf_tgetblk(sb, block);
> 937         lock_buffer(epos.bh);
> 938         memset(epos.bh->b_data, 0x00, bsize);
> 939         set_buffer_uptodate(epos.bh);
> 940         unlock_buffer(epos.bh);

At line 936 epos.bh is assigned with the value returned by udf_tgetblk. Then, epos.bh is used without any check, however it could be NULL.
The function udf_tgetblk is defined in udf/misc.c and returns the value of sb_getblk function without any check.
> 31 struct buffer_head *udf_tgetblk(struct super_block *sb, udf_pblk_t block)
> 32 {
> 33     if (UDF_QUERY_FLAG(sb, UDF_FLAG_VARCONV))
> 34         return sb_getblk(sb, udf_fixed_to_variable(block));
> 35     else
> 36         return sb_getblk(sb, block);
> 37 }

Considering the older patches related to sb_getblk, a possible fix is the following:
> 936         epos.bh = udf_tgetblk(sb, block);
> +++         if (unlikely(!epos.bh)) {
> +++             err = -ENOMEM;
> +++             goto out_no_entry;
> +++         }
> 937         lock_buffer(epos.bh);
> 938         memset(epos.bh->b_data, 0x00, bsize);
> 939         set_buffer_uptodate(epos.bh);
> 940         unlock_buffer(epos.bh);