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authorDavid Miller <davem@davemloft.net>2007-11-07 00:13:56 -0500
committerLinus Torvalds <torvalds@woody.linux-foundation.org>2007-11-09 19:13:08 -0500
commit3c5fd9c77d609b51c0bab682c9d40cbb496ec6f1 (patch)
tree72f8be799a5629458aca1da877d7616d928fa00d /kernel/user.c
parent8fc543c8f004fc9dfe0a262dc452dfe2eca4589b (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')
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