linux-kernel - Re: [PATCH?] reiserfs: prevent panic: don't allow %-char in journal dev. name

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Date:   Fri, 6 Apr 2018 15:45:14 +0200
From:   Rasmus Villemoes <linux@...musvillemoes.dk>
To:     Rasmus Villemoes <linux@...musvillemoes.dk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Randy Dunlap <rdunlap@...radead.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        reiserfs-devel@...r.kernel.org,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Jeff Mahoney <jeffm@...e.com>, Jan Kara <jack@...e.com>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Artem Bityutskiy <dedekind1@...il.com>
Subject: Re: [PATCH?] reiserfs: prevent panic: don't allow %-char in journal
 dev. name

On 2018-04-05 11:04, Rasmus Villemoes wrote:
> On 2018-04-05 03:45, Andrew Morton wrote:
>>
>> Isn't the bug in journal_init_dev()?
> 
> Urgh. At first I was about to reply that the real bug was in reiserfs.h
> for failing to annotate __reiserfs_warning with __printf(). But digging
> into it, it turns out that it implements its own printf extensions, so
> that's obviously a non-starter. Now, one thing is that some of those
> extension clash with existing standard modifiers (%z and %h, so if
> someone adds a correct %zu thing to print a size_t in reiserfs things
> will break). But, and I hope I'm wrong about this and just hasn't had
> enough coffee, this seems completely broken:
> 
>         while ((k = is_there_reiserfs_struct(fmt1, &what)) != NULL) {
>                 *k = 0;
> 
>                 p += vsprintf(p, fmt1, args);
> 
>                 switch (what) {
>                 case 'k':
>                         sprintf_le_key(p, va_arg(args, struct
> reiserfs_key *));
>                         break;
> 
> On architectures where va_list is a typedef for a one-element array of
> some struct (x86-64), that works ok, because the vsprintf call can and
> does update the args metadata. But when args is just a pointer into the
> stack (i386), we don't know how much vsprintf consumed, and end up
> consuming the same arguments again - only this time we may interpret
> some random integer as a struct pointer...

OK, so maybe -mregparm=3 would be the thing making i386 behave like
x86-64 wrt. varargs, but no, when calling a variadic function, gcc
pushes all arguments on the stack, and va_list is still just a pointer
(passed by value to vsprintf) into the stack.

It is only a problem when the format string contains ordinary specifiers
before a reiserfs-specific one, and such calls happen to be rare, but
not non-existing. One example would be reiserfs_warning(tb->tb_sb,
"vs-12339", "%s (%b)", which, bh);. Ok, treating which as a buffer_head
would probably just give some garbage numbers. But

  "reiserfs-16100", "STATDATA, index %d, type 0x%x, %h",		 vi->vi_index,
vi->vi_type, vi->vi_ih

ends up treating vi->vi_index as a struct item_head*, no?

Rasmus