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MIPS: cevt-r4k: Fix out-of-bounds array access
commit 9d7f29c upstream. calculate_min_delta() may incorrectly access a 4th element of buf2[] which only has 3 elements. This may trigger undefined behaviour and has been reported to cause strange crashes in start_kernel() sometime after timer initialization when built with GCC 5.3, possibly due to register/stack corruption: sched_clock: 32 bits at 200MHz, resolution 5ns, wraps every 10737418237ns CPU 0 Unable to handle kernel paging request at virtual address ffffb0aa, epc == 8067daa8, ra == 8067da84 Oops[hardkernel#1]: CPU: 0 PID: 0 Comm: swapper/0 Not tainted 4.9.18 hardkernel#51 task: 8065e3e0 task.stack: 80644000 $ 0 : 00000000 00000001 00000000 00000000 $ 4 : 8065b4d0 00000000 805d0000 00000010 $ 8 : 00000010 80321400 fffff000 812de408 $12 : 00000000 00000000 00000000 ffffffff $16 : 00000002 ffffffff 80660000 806a666c $20 : 806c0000 00000000 00000000 00000000 $24 : 00000000 00000010 $28 : 80644000 80645ed0 00000000 8067da84 Hi : 00000000 Lo : 00000000 epc : 8067daa8 start_kernel+0x33c/0x500 ra : 8067da84 start_kernel+0x318/0x500 Status: 11000402 KERNEL EXL Cause : 4080040c (ExcCode 03) BadVA : ffffb0aa PrId : 0501992c (MIPS 1004Kc) Modules linked in: Process swapper/0 (pid: 0, threadinfo=80644000, task=8065e3e0, tls=00000000) Call Trace: [<8067daa8>] start_kernel+0x33c/0x500 Code: 24050240 0c0131f9 24849c64 <a200b0a8> 41606020 000000c0 0c1a45e6 00000000 0c1a5f44 UBSAN also detects the same issue: ================================================================ UBSAN: Undefined behaviour in arch/mips/kernel/cevt-r4k.c:85:41 load of address 80647e4c with insufficient space for an object of type 'unsigned int' CPU: 0 PID: 0 Comm: swapper/0 Not tainted 4.9.18 hardkernel#47 Call Trace: [<80028f70>] show_stack+0x88/0xa4 [<80312654>] dump_stack+0x84/0xc0 [<8034163c>] ubsan_epilogue+0x14/0x50 [<803417d8>] __ubsan_handle_type_mismatch+0x160/0x168 [<8002dab0>] r4k_clockevent_init+0x544/0x764 [<80684d34>] time_init+0x18/0x90 [<8067fa5c>] start_kernel+0x2f0/0x500 ================================================================= buf2[] is intentionally only 3 elements so that the last element is the median once 5 samples have been inserted, so explicitly prevent the possibility of comparing against the 4th element rather than extending the array. Fixes: 1fa4055 ("MIPS: cevt-r4k: Dynamically calculate min_delta_ns") Reported-by: Rabin Vincent <[email protected]> Signed-off-by: James Hogan <[email protected]> Tested-by: Rabin Vincent <[email protected]> Cc: [email protected] Patchwork: https://patchwork.linux-mips.org/patch/15892/ Signed-off-by: Ralf Baechle <[email protected]> Signed-off-by: Greg Kroah-Hartman <[email protected]>
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