In the Linux kernel, the following vulnerability has been resolved:<\/p>\n
bpf: Infer zext_dst based on static register liveness analysis<\/p>\n
As reported in the thread [1], the verifier’s 32-bit operations zero
\nextension logic is broken. This logic is responsible for correct
\nsemantics of 32-bit operations on s390 architecture.<\/p>\n
According to BPF semantics, operation `w1 += 1` is supposed to zero
\nextend the upper half of the register `r1`. On s390 the JIT relies on
\nthe verifier emitting explicit zero extension before such operations.<\/p>\n
The verifier attempts to minimize the amount of zero extensions
\ninserted by tracking whether upper halves of the 64-bit registers are
\never used. Previously such tracking worked as follows:<\/p>\n
– bpf_reg_state->subreg_def field was set by do_check_insn()
\n for each operation defining lower but not the upper halves
\n of the register.
\n– Whenever an operation reading the whole register was verified,
\n the verifier checked register’s subreg_def and set
\n bpf_insn_aux_data->zext_dst flag as true via a call to
\n mark_insn_zext() function.
\n– After the verification was complete, a special pass
\n bpf_opt_subreg_zext_lo32_rnd_hi32() extended 32-bit operations
\n with bpf_insn_aux_data->zext_dst set as true by adding
\n explicit zero extension.<\/p>\n
Note that the logic above relies on bpf_reg_state->subreg_def,
\nwhich is a property of a current verifier state.
\nBefore the commit [2] two additional steps happened:<\/p>\n
– The verifier tracked upper and lower register halves’ liveness as
\n flags REG_LIVE_READ{32,64} in bpf_reg_state->live.
\n– The function propagate_liveness() called mark_insn_zext()
\n in order to transfer the knowledge about which registers have
\n their upper halves alive (and thus might require zero extension).<\/p>\n
The commit [2] removed the two steps described above,
\nhence making possible a situation like below:<\/p>\n
– The register’s upper half is set and is used on some verification
\n path P1 and the register happens not to be marked as precise.
\n– The checkpoint C is created while processing some instruction
\n between register initialization and usage.
\n– On some other verification path P2 the register’s upper half is not
\n initialized and that path ends hitting the checkpoint C.
\n– In such a case the register’s initialization on path P2 would lack
\n zext_dst mark, making it possible for the program to inject
\n an arbitrary value in the register’s upper half.<\/p>\n
This commit replaces subreg_def based logic with computing zext_dst
\nstatically, as a part of the bpf_compute_live_registers() analysis:<\/p>\n
– The analysis now tracks usage of upper and lower halves of the
\n registers separately.
\n– If some instruction defines a 32-bit subregister, but not the whole
\n register, *and* the upper half of the register is alive after that
\n instruction, the instruction is marked as zext_dst.<\/p>\n
There is one notable drop in precision: whenever a BPF subprogram is
\ncalled, all 64 bits of parameter registers are presumed to be used.
\nThe assumption is that such a drop in precision would not inflict
\na noticeable performance penalty.<\/p>\n
[1] https:\/\/lore.kernel.org\/bpf\/CAGKGUv=sOuqQtA1Ub-5JXfA4FPosJFYKAQE4B79cK+P1erxqtg@mail.gmail.com\/
\n[2] commit 107e16979905 (“bpf: disable and remove registers chain based liveness”)<\/p>\n","protected":false},"excerpt":{"rendered":"
In the Linux kernel, the following vulnerability has been resolved:<\/p>\n
bpf: Infer zext_dst based on static register liveness analysis<\/p>\n
As reported in the thread [1], the verifier’s 32-bit operations zer<\/p>\n","protected":false},"template":"","meta":[],"categories":[],"tags":[],"class_list":["post-44129","vuln","type-vuln","status-publish","hentry"],"_links":{"self":[{"href":"https:\/\/www.54iter.cn\/index.php?rest_route=\/wp\/v2\/vuln\/44129","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=44129"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.54iter.cn\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=44129"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.54iter.cn\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=44129"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}