From mboxrd@z Thu Jan 1 00:00:00 1970 From: Mitchell Blank Jr Date: Sat, 01 Apr 2006 02:35:38 +0000 Subject: Re: [patch] avoid unaligned access when accessing poll stack Message-Id: <20060401023538.GB3157@gaz.sfgoth.com> List-Id: References: <200603311800.19364.ak@suse.de> In-Reply-To: MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: Jes Sorensen Cc: Andi Kleen , Linus Torvalds , Andrew Morton , linux-kernel@vger.kernel.org, linux-ia64@vger.kernel.org Jes Sorensen wrote: > I assume you mean select(). > > Updated patch attached. This fixes a few problems introduced by this patch. * Fixes two warnings caused by mixing "char *" and "long *" pointers * If SELECT_STACK_ALLOC is not a multiple of sizeof(long) then stack_fds[] would be less than SELECT_STACK_ALLOC bytes and could overflow later in the function. Fixed by simply rearranging the test later to work on sizeof(stack_fds) Currently SELECT_STACK_ALLOC is 256 so this doesn't happen, but it's nasty to have things like this hidden in the code. What if later someone decides to change SELECT_STACK_ALLOC to 300? * I also changed "size" to be unsigned since that makes more sense and is less prone to overflow bugs. I'm also a little scared by the "kmalloc(6 * size)" since that type of call is a classic multiply-overflow security hole (hence libc's calloc() API). However "size" is constrained by fdt->max_fdset so I think it isn't exploitable. The kernel doesn't have an overflow-safe API for kmalloc'ing arrays, does it? Patch is vs current git HEAD. Only compile/boot tested. Signed-off-by: Mitchell Blank Jr diff --git a/fs/select.c b/fs/select.c index 071660f..bd9c7db 100644 --- a/fs/select.c +++ b/fs/select.c @@ -311,7 +311,8 @@ static int core_sys_select(int n, fd_set { fd_set_bits fds; char *bits; - int ret, size, max_fdset; + int ret, max_fdset; + unsigned int size; struct fdtable *fdt; /* Allocate small arguments on the stack to save memory and be faster */ long stack_fds[SELECT_STACK_ALLOC/sizeof(long)]; @@ -335,8 +336,8 @@ static int core_sys_select(int n, fd_set */ ret = -ENOMEM; size = FDS_BYTES(n); - if (6*size < SELECT_STACK_ALLOC) - bits = stack_fds; + if (size < sizeof(stack_fds) / 6) + bits = (char *) stack_fds; else bits = kmalloc(6 * size, GFP_KERNEL); if (!bits) @@ -373,7 +374,7 @@ static int core_sys_select(int n, fd_set ret = -EFAULT; out: - if (bits != stack_fds) + if (bits != (char *) stack_fds) kfree(bits); out_nofds: return ret;