diff options
author | David Miller <davem@davemloft.net> | 2007-11-07 00:13:56 -0500 |
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committer | Linus Torvalds <torvalds@woody.linux-foundation.org> | 2007-11-09 19:13:08 -0500 |
commit | 3c5fd9c77d609b51c0bab682c9d40cbb496ec6f1 (patch) | |
tree | 72f8be799a5629458aca1da877d7616d928fa00d /kernel/user.c | |
parent | 8fc543c8f004fc9dfe0a262dc452dfe2eca4589b (diff) |
[FUTEX] Fix address computation in compat code.
compat_exit_robust_list() computes a pointer to the
futex entry in userspace as follows:
(void __user *)entry + futex_offset
'entry' is a 'struct robust_list __user *', and
'futex_offset' is a 'compat_long_t' (typically a 's32').
Things explode if the 32-bit sign bit is set in futex_offset.
Type promotion sign extends futex_offset to a 64-bit value before
adding it to 'entry'.
This triggered a problem on sparc64 running 32-bit applications which
would lock up a cpu looping forever in the fault handling for the
userspace load in handle_futex_death().
Compat userspace runs with address masking (wherein the cpu zeros out
the top 32-bits of every effective address given to a memory operation
instruction) so the sparc64 fault handler accounts for this by
zero'ing out the top 32-bits of the fault address too.
Since the kernel properly uses the compat_uptr interfaces, kernel side
accesses to compat userspace work too since they will only use
addresses with the top 32-bit clear.
Because of this compat futex layer bug we get into the following loop
when executing the get_user() load near the top of handle_futex_death():
1) load from address '0xfffffffff7f16bd8', FAULT
2) fault handler clears upper 32-bits, processes fault
for address '0xf7f16bd8' which succeeds
3) goto #1
I want to thank Bernd Zeimetz, Josip Rodin, and Fabio Massimo Di Nitto
for their tireless efforts helping me track down this bug.
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Diffstat (limited to 'kernel/user.c')
0 files changed, 0 insertions, 0 deletions