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personality handling: fix PER_CLEAR_ON_SETID for security reasons

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We have found that the current PER_CLEAR_ON_SETID mask on Linux
doesn't include neither ADDR_COMPAT_LAYOUT, nor MMAP_PAGE_ZERO.

The current mask is READ_IMPLIES_EXEC|ADDR_NO_RANDOMIZE.

We believe it is important to add MMAP_PAGE_ZERO, because by using
this personality it is possible to have the first page mapped inside a
process running as setuid root. This could be used in those scenarios:

- Exploiting a NULL pointer dereference issue in a setuid root binary
- Bypassing the mmap_min_addr restrictions of the Linux kernel: by
running a setuid binary that would drop privileges before giving us
control back (for instance by loading a user-supplied library), we
could get the first page mapped in a process we control. By further
using mremap and mprotect on this mapping, we can then completely
bypass the mmap_min_addr restrictions.

Less importantly, we believe ADDR_COMPAT_LAYOUT should also be added
since on x86 32bits it will in practice disable most of the address
space layout randomization (only the stack will remain randomized).

Signed-off-by: Julien Tinnes <jt@cr0.org>
Signed-off-by: Tavis Ormandy <taviso@sdf.lonestar.org>
Acked-by: Christoph Hellwig <hch@infradead.org>
Acked-by: Kees Cook <kees.cook@canonical.com>
Signed-off-by: James Morris <jmorris@namei.org>
This commit is contained in:
Julien Tinnes 2009-07-10 10:46:30 -07:00 committed by James Morris
parent 8113a8d80f
commit b3a633c852
1 changed files with 5 additions and 1 deletions
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6
include/linux/personality.h
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@ -40,7 +40,11 @@ enum {
* Security-relevant compatibility flags that must be * Security-relevant compatibility flags that must be
* cleared upon setuid or setgid exec: * cleared upon setuid or setgid exec:
*/ */
#define PER_CLEAR_ON_SETID (READ_IMPLIES_EXEC|ADDR_NO_RANDOMIZE) #define PER_CLEAR_ON_SETID \
(READ_IMPLIES_EXEC | \
ADDR_NO_RANDOMIZE | \
ADDR_COMPAT_LAYOUT | \
MMAP_PAGE_ZERO)
/* /*
* Personality types. * Personality types.