From 27bfa95847f7addd4ec49e41ea2a916aeadb9418 Mon Sep 17 00:00:00 2001 From: "Simon L. B. Nielsen" Date: Sun, 27 Sep 2009 14:49:51 +0000 Subject: [PATCH] Do not allow mmap with the MAP_FIXED argument to map at address zero. This is done to make it harder to exploit kernel NULL pointer security vulnerabilities. While this of course does not fix vulnerabilities, it does mitigate their impact. Note that this may break some applications, most likely emulators or similar, which for one reason or another require mapping memory at zero. This restriction can be disabled with the security.bsd.mmap_zero sysctl variable. Discussed with: rwatson, bz Tested by: bz (Wine), simon (VirtualBox) Submitted by: jhb --- sys/vm/vm_mmap.c | 19 ++++++++++++++++++- 1 file changed, 18 insertions(+), 1 deletion(-) diff --git a/sys/vm/vm_mmap.c b/sys/vm/vm_mmap.c index c8d25ee3a305..e8ba13415a7b 100644 --- a/sys/vm/vm_mmap.c +++ b/sys/vm/vm_mmap.c @@ -96,6 +96,14 @@ static int max_proc_mmap; SYSCTL_INT(_vm, OID_AUTO, max_proc_mmap, CTLFLAG_RW, &max_proc_mmap, 0, "Maximum number of memory-mapped files per process"); +/* + * 'mmap_zero' determines whether or not MAP_FIXED mmap() requests for + * virtual address zero are permitted. + */ +static int mmap_zero; +SYSCTL_INT(_security_bsd, OID_AUTO, mmap_zero, CTLFLAG_RW, &mmap_zero, 0, + "Processes may map an object at virtual address zero"); + /* * Set the maximum number of vm_map_entry structures per process. Roughly * speaking vm_map_entry structures are tiny, so allowing them to eat 1/100 @@ -229,7 +237,8 @@ mmap(td, uap) pos = uap->pos; fp = NULL; - /* make sure mapping fits into numeric range etc */ + + /* Make sure mapping fits into numeric range, etc. */ if ((uap->len == 0 && !SV_CURPROC_FLAG(SV_AOUT) && curproc->p_osrel >= 800104) || ((flags & MAP_ANON) && uap->fd != -1)) @@ -267,6 +276,14 @@ mmap(td, uap) addr -= pageoff; if (addr & PAGE_MASK) return (EINVAL); + + /* + * Mapping to address zero is only permitted if + * mmap_zero is enabled. + */ + if (addr == 0 && !mmap_zero) + return (EINVAL); + /* Address range must be all in user VM space. */ if (addr < vm_map_min(&vms->vm_map) || addr + size > vm_map_max(&vms->vm_map))