In the Linux kernel, the following vulnerability has been resolved:<\/p>\n
bpf, x86: Fix per-CPU address resolution into an extended register<\/p>\n
The destination of the per-CPU address MOV is encoded in ModRM.reg,
\nwhich is extended by REX.R, but the REX prefix is built with
\nadd_1mod(), which sets REX.B. REX.B extends ModRM.rm and SIB.base, and
\nthis instruction addresses memory as disp32 with no base, so the bit
\nhas no effect at all and the high register bit is simply lost.<\/p>\n
Every is_ereg() destination therefore resolves to the wrong register,
\npicking whichever one shares the low three bits:<\/p>\n
R5 -> RAX R7 -> RBP R8 -> RSI R9 -> RDI<\/p>\n
With BPF_REG_5, whose reg2hex is 0, the emitted<\/p>\n
65 49 03 04 25 \tadd %gs:,%rax<\/p>\n
adds the per-CPU offset to RAX rather than R8. The destination keeps
\nthe unadjusted address and RAX is clobbered, so the program goes on to
\ndereference a pointer that was never made per-CPU:<\/p>\n
BUG: unable to handle page fault for address: 0000607e386a8894
\n RIP: bpf_prog_707837aafd2aa9ae_update_percpu_data+0x93\/0xc9
\n Call Trace:
\n __bpf_prog_test_run_raw_tp+0x2dc\/0x7d0
\n __flush_smp_call_function_queue+0x1e9\/0xc80
\n Kernel panic – not syncing: Fatal exception in interrupt<\/p>\n
R5 is the mildest of the four, aliasing a scratch register and faulting
\nat the store. R7 aliases RBP and would corrupt the frame pointer, R8
\nand R9 alias the argument registers.<\/p>\n
Use add_2mod() so the register goes through REX.R, matching how
\nadd_2reg() places it in ModRM.reg and how emit_priv_frame_ptr()
\nhardcodes 0x4c for the same instruction with R9. Encodings for the
\nnon-extended registers are unchanged.<\/p>\n
Problem showed up when trying to resurrect BPF_GCC CI (selftests built
\nwith BPF_GCC).<\/p>\n
This has gone unnoticed because clang reloads the address into R1
\nbefore each per-CPU access, so the destination is never an extended
\nregister. GCC keeps several per-CPU addresses live at once, and
\ntest_progs-bpf_gcc panics the kernel in global_percpu_data\/init, where
\nthe address of a .percpu variable ends up in R5.<\/p>\n","protected":false},"excerpt":{"rendered":"
In the Linux kernel, the following vulnerability has been resolved:<\/p>\n
bpf, x86: Fix per-CPU address resolution into an extended register<\/p>\n
The destination of the per-CPU address MOV is encoded in ModRM.<\/p>\n","protected":false},"template":"","meta":[],"categories":[],"tags":[],"class_list":["post-21503","vuln","type-vuln","status-publish","hentry"],"_links":{"self":[{"href":"https:\/\/www.54iter.cn\/index.php?rest_route=\/wp\/v2\/vuln\/21503","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.54iter.cn\/index.php?rest_route=\/wp\/v2\/vuln"}],"about":[{"href":"https:\/\/www.54iter.cn\/index.php?rest_route=\/wp\/v2\/types\/vuln"}],"wp:attachment":[{"href":"https:\/\/www.54iter.cn\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=21503"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.54iter.cn\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=21503"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.54iter.cn\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=21503"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}