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Date: Sun, 29 Dec 2013 01:01:12 +0100
From: Szabolcs Nagy <nsz@...t70.net>
To: musl@...ts.openwall.com
Subject: Re: Re: NULL deref SEGV in malloc.c:unbin()
* David Wuertele <dave+gmane@...rtele.com> [2013-12-28 01:28:42 +0000]:
> I removed all my calls to malloc(), now it is only musl that is
> calling it. I'm watching everything that I think might be related.
> It looks like calloc(), opendir(), or free() are the culprit.
>
> (gdb) info watch
> Num Type Disp Enb Address What
> 2 watchpoint keep y mal.bins[40].head
> breakpoint already hit 2 times
> print *mal.bins[40].head
> bt
> cont
> 3 watchpoint keep y mal.bins[40].head.next
> breakpoint already hit 47 times
> print *mal.bins[40].head
> bt
> cont
> 3.1 y mal.bins[40].head.next
> 3.2 y mal.bins[40].head.next
> 4 watchpoint keep y mal.bins[40].head.prev
> breakpoint already hit 32 times
> print *mal.bins[40].head
> bt
> cont
> 4.1 y mal.bins[40].head.prev
> 4.2 y mal.bins[40].head.prev
> (gdb) c
>
> Watchpoint 4: mal.bins[40].head.prev
>
> Old value = (struct chunk *) 0xc9690
> New value = (struct chunk *) 0x0
> 0x0008b268 in calloc (m=519, n=2080) at src/malloc/calloc.c:20
> 20 in src/malloc/calloc.c
> $78 = {psize = 2097, csize = 2097, next = 0x0, prev = 0x0}
> #0 0x0008b268 in calloc (m=519, n=2080) at src/malloc/calloc.c:20
> #1 0x00089f30 in opendir (name=0xaa8d4 ".") at src/dirent/opendir.c:19
> #2 0x0000fb50 in mkchdir (path=0xbe83f89b "") at prog.c:1845
if i understand the code correctly the state is already corrupted here
the printed head chunk is returned from malloc (calloc zeros out next/prev)
but it is still in the free list (bin[40].head)
after the last free chunk is used up in a bin i think
head == tail == BIN_TO_CHUNK(i) should hold
(and mal.binmap & (1ULL<<i) should be 0)
> Watchpoint 3: mal.bins[40].head.next
>
> Old value = (struct chunk *) 0x0
> New value = (struct chunk *) 0x10
> opendir (name=0xaa8d4 ".") at src/dirent/opendir.c:24
> 24 src/dirent/opendir.c: No such file or directory.
> in src/dirent/opendir.c
> $79 = {psize = 2097, csize = 2097, next = 0x10, prev = 0x0}
> #0 opendir (name=0xaa8d4 ".") at src/dirent/opendir.c:24
> #1 0x0000fb50 in mkchdir (path=0xbe83f89b "") at prog.c:1845
this is just opendir setting dir->fd (dir == &head->next)
> Watchpoint 3: mal.bins[40].head.next
>
> Old value = (struct chunk *) 0x10
> New value = (struct chunk *) 0x2139018
> free (p=0x2139020) at src/malloc/malloc.c:530
> 530 src/malloc/malloc.c: No such file or directory.
> in src/malloc/malloc.c
> $80 = {psize = 2096, csize = 2097, next = 0x2139018, prev = 0x0}
> #0 free (p=0x2139020) at src/malloc/malloc.c:530
> #1 0x00089e84 in closedir (dir=0x2139020) at src/dirent/closedir.c:9
> #2 0x0000fc80 in mkchdir (path=0xd22bc "/var/spool") at prog.c:1873
a chunk is freed here (0x2139018) which we haven't seen yet
but is put on the same free list that is already broken
self->prev->next = self;
triggers the watchpoint (where &self->prev->next == &bin[40].head)
so the free-list has two chunks now: head is the broken chunk
returned in opendir and the new one freed here
>
> Program received signal SIGSEGV, Segmentation fault.
> 0x0008be70 in unbin (c=0x2139848, i=40) at src/malloc/malloc.c:241
> 241 in src/malloc/malloc.c
> (gdb)
>
i assume 0x2139848 is the broken chunk and unbin stumbles
on the 0 prev pointer
so it seems the corruption starts before opendir
it would be nice to see where 0x2139018 comes from and why
mal.binmap and mal.bin[40] aren't managed properly
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