While building coreutils on trusty with the testsuite enabled, we hit the following panic reproducibly on our Mustang buildds:

[  715.969679] Kernel panic - not syncing: BUG!
[  715.973930] CPU: 1 PID: 10872 Comm: bash Not tainted 3.13.0-57-generic #95-Ubuntu
[  715.981373] Call trace:
[  715.983811] [<ffffffc00008848c>] dump_backtrace+0x0/0x16c
[  715.989182] [<ffffffc000088608>] show_stack+0x10/0x1c
[  715.994208] [<ffffffc00060c660>] dump_stack+0x74/0x94
[  715.999232] [<ffffffc000606864>] panic+0xe0/0x20c
[  716.003911] [<ffffffc00017792c>] __get_user_pages+0x300/0x4ec
[  716.009625] [<ffffffc000177b58>] get_user_pages+0x40/0x4c
[  716.014996] [<ffffffc0001ad6d0>] get_arg_page+0x44/0xb8
[  716.020193] [<ffffffc0001ae0e0>] copy_strings.isra.21+0x148/0x268
[  716.026254] [<ffffffc0001aee90>] do_execve_common.isra.25+0x3fc/0x5b4
[  716.032661] [<ffffffc0001af064>] do_execve+0x1c/0x28
[  716.037599] [<ffffffc0001af2c0>] SyS_execve+0x34/0x54
[  716.042624] CPU0: stopping
[  716.045317] CPU: 0 PID: 0 Comm: swapper/0 Not tainted 3.13.0-57-generic #95-Ubuntu
[  716.052846] Call trace:
[  716.055280] [<ffffffc00008848c>] dump_backtrace+0x0/0x16c
[  716.060650] [<ffffffc000088608>] show_stack+0x10/0x1c
[  716.065674] [<ffffffc00060c660>] dump_stack+0x74/0x94
[  716.070699] [<ffffffc00008ed3c>] handle_IPI+0x120/0x138
[  716.075895] [<ffffffc0000812cc>] gic_handle_irq+0x74/0x7c
[  716.081264] Exception stack(0xffffffc0008efde0 to 0xffffffc0008eff00)
[  716.087672] fde0: 008ec000 ffffffc0 008ec000 ffffffc0 008eff20 ffffffc0 00085600 ffffffc0
[  716.095808] fe00: 0005ae0c 00000000 00000000 00000000 fff80834 ffffffc3 0000a281 00000001
[  716.103942] fe20: 00000020 00000000 00000010 00000000 b9191580 000000a6 fff80df0 ffffffc3
[  716.112077] fe40: 008f8e80 ffffffc0 008efd20 ffffffc0 0000a281 00000001 00000000 00000000
[  716.120211] fe60: 0000002d 00000000 004ed968 00000000 004ed000 00000000 ffffffff ffffffff
[  716.128345] fe80: 001af28c ffffffc0 004e2488 00000000 f00fad90 0000007f 008ec000 ffffffc0
[  716.136479] fea0: 008ec000 ffffffc0 00972000 ffffffc0 0098ac78 ffffffc0 00980198 ffffffc0
[  716.144613] fec0: 008ec000 ffffffc0 00971233 ffffffc0 0007d000 00000040 007d2890 ffffffc0
[  716.152749] fee0: 0061cd98 ffffffc0 008eff20 ffffffc0 000855fc ffffffc0 008eff20 ffffffc0
[  716.160883] [<ffffffc0000845a8>] el1_irq+0x68/0xc0
[  716.165648] [<ffffffc0000ed6e4>] cpu_startup_entry+0x12c/0x1e4
[  716.171451] [<ffffffc000603958>] rest_init+0x6c/0x78
[  716.176390] [<ffffffc00089d7e4>] start_kernel+0x350/0x368
[  716.181760] CPU7: stopping
[  716.184455] CPU: 7 PID: 0 Comm: swapper/7 Not tainted 3.13.0-57-generic #95-Ubuntu
[  716.191985] Call trace:
[  716.194420] [<ffffffc00008848c>] dump_backtrace+0x0/0x16c
[  716.199791] [<ffffffc000088608>] show_stack+0x10/0x1c
[  716.204815] [<ffffffc00060c660>] dump_stack+0x74/0x94
[  716.209840] [<ffffffc00008ed3c>] handle_IPI+0x120/0x138
[  716.215036] [<ffffffc0000812cc>] gic_handle_irq+0x74/0x7c
[  716.220405] Exception stack(0xffffffc3ec19be20 to 0xffffffc3ec19bf40)
[  716.226812] be20: ec198000 ffffffc3 ec198000 ffffffc3 ec19bf60 ffffffc3 00085600 ffffffc0
[  716.234946] be40: 0003bdee 00000000 00000000 00000000 fffcd834 ffffffc3 0000a281 00000001
[  716.243081] be60: 00000000 00000000 ff6e5000 00000003 00906140 ffffffc0 00000400 00000000
[  716.251215] be80: ec183c10 ffffffc3 ec19bd60 ffffffc3 0000a282 00000001 00000000 00000000
[  716.259350] bea0: 00000001 00000000 f00faf10 0000007f 0043a548 00000000 ffffffff ffffffff
[  716.267484] bec0: 000a92f8 ffffffc0 00115920 00000020 f00fad10 0000007f ec198000 ffffffc3
[  716.275618] bee0: ec198000 ffffffc3 00972000 ffffffc0 0098ac78 ffffffc0 00980198 ffffffc0
[  716.283752] bf00: ec198000 ffffffc3 00971233 ffffffc0 0007d000 00000040 007d2890 ffffffc0
[  716.291886] bf20: 0061cd98 ffffffc0 ec19bf60 ffffffc3 000855fc ffffffc0 ec19bf60 ffffffc3
[  716.300020] [<ffffffc0000845a8>] el1_irq+0x68/0xc0
[  716.304785] [<ffffffc0000ed6e4>] cpu_startup_entry+0x12c/0x1e4
[  716.310587] [<ffffffc00008e8b8>] secondary_start_kernel+0xf8/0x108
[  716.316734] CPU6: stopping
[  716.319427] CPU: 6 PID: 0 Comm: swapper/6 Not tainted 3.13.0-57-generic #95-Ubuntu
[  716.326956] Call trace:
[  716.329390] [<ffffffc00008848c>] dump_backtrace+0x0/0x16c
[  716.334760] [<ffffffc000088608>] show_stack+0x10/0x1c
[  716.339784] [<ffffffc00060c660>] dump_stack+0x74/0x94
[  716.344808] [<ffffffc00008ed3c>] handle_IPI+0x120/0x138
[  716.350004] [<ffffffc0000812cc>] gic_handle_irq+0x74/0x7c
[  716.355374] Exception stack(0xffffffc3ec197e20 to 0xffffffc3ec197f40)
[  716.361781] 7e20: ec194000 ffffffc3 ec194000 ffffffc3 ec197f60 ffffffc3 00085600 ffffffc0
[  716.369915] 7e40: 0003b8ac 00000000 00000000 00000000 fffc2834 ffffffc3 0000a281 00000001
[  716.378049] 7e60: 00000020 00000000 00000010 00000000 b9191580 000000a6 fffc2df0 ffffffc3
[  716.386184] 7e80: ec183110 ffffffc3 ec197d60 ffffffc3 0000a281 00000001 00000000 00000000
[  716.394318] 7ea0: 0001569d 00000000 f00fb230 0000007f 0043a548 00000000 00000000 00000000
[  716.402453] 7ec0: 000a92f8 ffffffc0 00115920 00000020 f00f9350 0000007f ec194000 ffffffc3
[  716.410587] 7ee0: ec194000 ffffffc3 00972000 ffffffc0 0098ac78 ffffffc0 00980198 ffffffc0
[  716.418721] 7f00: ec194000 ffffffc3 00971233 ffffffc0 0007d000 00000040 007d2890 ffffffc0
[  716.426855] 7f20: 0061cd98 ffffffc0 ec197f60 ffffffc3 000855fc ffffffc0 ec197f60 ffffffc3
[  716.434989] [<ffffffc0000845a8>] el1_irq+0x68/0xc0
[  716.439754] [<ffffffc0000ed6e4>] cpu_startup_entry+0x12c/0x1e4
[  716.445555] [<ffffffc00008e8b8>] secondary_start_kernel+0xf8/0x108
[  716.451703] CPU5: stopping
[  716.454398] CPU: 5 PID: 0 Comm: swapper/5 Not tainted 3.13.0-57-generic #95-Ubuntu
[  716.461928] Call trace:
[  716.464363] [<ffffffc00008848c>] dump_backtrace+0x0/0x16c
[  716.469734] [<ffffffc000088608>] show_stack+0x10/0x1c
[  716.474759] [<ffffffc00060c660>] dump_stack+0x74/0x94
[  716.479782] [<ffffffc00008ed3c>] handle_IPI+0x120/0x138
[  716.484979] [<ffffffc0000812cc>] gic_handle_irq+0x74/0x7c
[  716.490348] Exception stack(0xffffffc3ec193e20 to 0xffffffc3ec193f40)
[  716.496755] 3e20: ec190000 ffffffc3 ec190000 ffffffc3 ec193f60 ffffffc3 00085600 ffffffc0
[  716.504891] 3e40: 0002ccb8 00000000 00000000 00000000 fffb7834 ffffffc3 0000a281 00000001
[  716.513025] 3e60: 00000020 00000000 00000010 00000000 26c54180 000000a7 fffb7df0 ffffffc3
[  716.521159] 3e80: ec182610 ffffffc3 ec193d60 ffffffc3 0000a279 00000001 00000000 00000000
[  716.529293] 3ea0: 00000020 00000000 00000018 00000000 ffffffff ffffffff ffffffff ffffffff
[  716.537427] 3ec0: 001af28c ffffffc0 004e2488 00000000 dfb530d0 0000007f ec190000 ffffffc3
[  716.545562] 3ee0: ec190000 ffffffc3 00972000 ffffffc0 0098ac78 ffffffc0 00980198 ffffffc0
[  716.553696] 3f00: ec190000 ffffffc3 00971233 ffffffc0 0007d000 00000040 007d2890 ffffffc0
[  716.561830] 3f20: 0061cd98 ffffffc0 ec193f60 ffffffc3 000855fc ffffffc0 ec193f60 ffffffc3
[  716.569963] [<ffffffc0000845a8>] el1_irq+0x68/0xc0
[  716.574728] [<ffffffc0000ed6e4>] cpu_startup_entry+0x12c/0x1e4
[  716.580530] [<ffffffc00008e8b8>] secondary_start_kernel+0xf8/0x108
[  716.586678] CPU2: stopping
[  716.589373] CPU: 2 PID: 0 Comm: swapper/2 Not tainted 3.13.0-57-generic #95-Ubuntu
[  716.596903] Call trace:
[  716.599338] [<ffffffc00008848c>] dump_backtrace+0x0/0x16c
[  716.604708] [<ffffffc000088608>] show_stack+0x10/0x1c
[  716.609733] [<ffffffc00060c660>] dump_stack+0x74/0x94
[  716.614757] [<ffffffc00008ed3c>] handle_IPI+0x120/0x138
[  716.619953] [<ffffffc0000812cc>] gic_handle_irq+0x74/0x7c
[  716.625322] Exception stack(0xffffffc3ec17fe20 to 0xffffffc3ec17ff40)
[  716.631729] fe20: ec17c000 ffffffc3 ec17c000 ffffffc3 ec17ff60 ffffffc3 00085600 ffffffc0
[  716.639864] fe40: 00032872 00000000 00000000 00000000 fff96834 ffffffc3 0000a281 00000001
[  716.647998] fe60: 00000020 00000000 00000010 00000000 b9191580 000000a6 fff96df0 ffffffc3
[  716.656132] fe80: ec180510 ffffffc3 ec17fd60 ffffffc3 0000a281 00000001 00000000 00000000
[  716.664266] fea0: 0001ec9b 00000000 00000040 00000000 004eb000 00000000 ffffffff ffffffff
[  716.672400] fec0: 001af28c ffffffc0 004e2488 00000000 f00f7d10 0000007f ec17c000 ffffffc3
[  716.680535] fee0: ec17c000 ffffffc3 00972000 ffffffc0 0098ac78 ffffffc0 00980198 ffffffc0
[  716.688669] ff00: ec17c000 ffffffc3 00971233 ffffffc0 0007d000 00000040 007d2890 ffffffc0
[  716.696804] ff20: 0061cd98 ffffffc0 ec17ff60 ffffffc3 000855fc ffffffc0 ec17ff60 ffffffc3
[  716.704939] [<ffffffc0000845a8>] el1_irq+0x68/0xc0
[  716.709704] [<ffffffc0000ed6e4>] cpu_startup_entry+0x12c/0x1e4
[  716.715506] [<ffffffc00008e8b8>] secondary_start_kernel+0xf8/0x108
[  716.721653] CPU3: stopping
[  716.724345] CPU: 3 PID: 0 Comm: swapper/3 Not tainted 3.13.0-57-generic #95-Ubuntu
[  716.731874] Call trace:
[  716.734308] [<ffffffc00008848c>] dump_backtrace+0x0/0x16c
[  716.739679] [<ffffffc000088608>] show_stack+0x10/0x1c
[  716.744703] [<ffffffc00060c660>] dump_stack+0x74/0x94
[  716.749727] [<ffffffc00008ed3c>] handle_IPI+0x120/0x138
[  716.754923] [<ffffffc0000812cc>] gic_handle_irq+0x74/0x7c
[  716.760292] Exception stack(0xffffffc3ec18be20 to 0xffffffc3ec18bf40)
[  716.766699] be20: ec188000 ffffffc3 ec188000 ffffffc3 ec18bf60 ffffffc3 00085600 ffffffc0
[  716.774834] be40: 00039a32 00000000 00000000 00000000 fffa1834 ffffffc3 0000a281 00000001
[  716.782968] be60: 00000000 00000000 fffa1c90 ffffffc3 b9191580 000000a6 e772fd48 ffffffc3
[  716.791103] be80: ec181010 ffffffc3 ec18bd60 ffffffc3 0000a27a 00000001 00000000 00000000
[  716.799237] bea0: 00000018 00000000 e8000000 00000003 00000000 00000000 d3fef76e 0021ced1
[  716.807373] bec0: 001ef6a4 ffffffc0 b55e6360 0000007f a3ffe560 0000007f ec188000 ffffffc3
[  716.815507] bee0: ec188000 ffffffc3 00972000 ffffffc0 0098ac78 ffffffc0 00980198 ffffffc0
[  716.823642] bf00: ec188000 ffffffc3 00971233 ffffffc0 0007d000 00000040 007d2890 ffffffc0
[  716.831776] bf20: 0061cd98 ffffffc0 ec18bf60 ffffffc3 000855fc ffffffc0 ec18bf60 ffffffc3
[  716.839911] [<ffffffc0000845a8>] el1_irq+0x68/0xc0
[  716.844675] [<ffffffc0000ed6e4>] cpu_startup_entry+0x12c/0x1e4
[  716.850477] [<ffffffc00008e8b8>] secondary_start_kernel+0xf8/0x108
[  716.856624] CPU4: stopping
[  716.859317] CPU: 4 PID: 0 Comm: swapper/4 Not tainted 3.13.0-57-generic #95-Ubuntu
[  716.866846] Call trace:
[  716.869280] [<ffffffc00008848c>] dump_backtrace+0x0/0x16c
[  716.874651] [<ffffffc000088608>] show_stack+0x10/0x1c
[  716.879675] [<ffffffc00060c660>] dump_stack+0x74/0x94
[  716.884699] [<ffffffc00008ed3c>] handle_IPI+0x120/0x138
[  716.889895] [<ffffffc0000812cc>] gic_handle_irq+0x74/0x7c
[  716.895264] Exception stack(0xffffffc3ec18fe20 to 0xffffffc3ec18ff40)
[  716.901671] fe20: ec18c000 ffffffc3 ec18c000 ffffffc3 ec18ff60 ffffffc3 00085600 ffffffc0
[  716.909807] fe40: 00032884 00000000 00000000 00000000 fffac834 ffffffc3 0000a281 00000001
[  716.917941] fe60: 00000020 00000000 00000010 00000000 b9191580 000000a6 00984d18 ffffffc0
[  716.926075] fe80: ec181b10 ffffffc3 ec18fd60 ffffffc3 00000400 00000000 00000000 00000000
[  716.934210] fea0: 00012314 00000000 dfb53250 0000007f 0043a548 00000000 ffffffff ffffffff
[  716.942344] fec0: 000a92f8 ffffffc0 00115920 00000020 dfb51c30 0000007f ec18c000 ffffffc3
[  716.950478] fee0: ec18c000 ffffffc3 00972000 ffffffc0 0098ac78 ffffffc0 00980198 ffffffc0
[  716.958612] ff00: ec18c000 ffffffc3 00971233 ffffffc0 0007d000 00000040 007d2890 ffffffc0
[  716.966747] ff20: 0061cd98 ffffffc0 ec18ff60 ffffffc3 000855fc ffffffc0 ec18ff60 ffffffc3
[  716.974882] [<ffffffc0000845a8>] el1_irq+0x68/0xc0
[  716.979647] [<ffffffc0000ed6e4>] cpu_startup_entry+0x12c/0x1e4
[  716.985449] [<ffffffc00008e8b8>] secondary_start_kernel+0xf8/0x108