In the Linux kernel, the following vulnerability has been resolved:<\/p>\n
bpf: check_cond_jmp_op(): properly infer if register is null<\/p>\n
Nicholas Carlini reported a bug when verifier can incorrectly infer
\nthat a pointer is non-null. The bug occurs when two pointers are
\ncompared and one of them has a type w\/o PTR_MAYBE_NULL flag,
\nbut which allows a value to be NULL at runtime.
\nHere is an example:<\/p>\n
\/\/ `a` is PTR_TO_MEM | MEM_RDONLY | PTR_UNTRUSTED
\n \/\/ `a` is 0 at runtime.
\n \/\/ `b` is PTR_TO_MAP_VALUE | PTR_MAYBE_NULL
\n void *a = bpf_rdonly_cast(0, 0);
\n int *b = bpf_map_lookup_elem(…);<\/p>\n
if (a == b)
\n *b = 42; \/\/ verifier does not catch null pointer dereference<\/p>\n
This happens because of a special case in check_cond_jmp_op(),
\nwhich attempts to strip PTR_MAYBE_NULL flags from pointer types,
\nwhen processing comparisons like `rA == rB`, if either rA or rB can’t
\nbe null.<\/p>\n
The non-null property is derived based on the absence of
\nPTR_MAYBE_NULL flag on rA’s or rB’s type. But that is not sufficient
\nfor types like PTR_TO_MEM, as in the example.<\/p>\n
This patch replaces type_may_be_null() call with reg_not_null(),
\nwhich contains an allowlist of types for which absence of
\nPTR_MAYBE_NULL actually means that the value can’t be NULL at runtime.<\/p>\n
At the moment, the list in the reg_not_null() omits two types for
\nwhich PTR_MAYBE_NULL is applicable: PTR_TO_XDP_SOCK and PTR_TO_BUF.
\nIn order to remain backward compatible, and assuming that only
\ncomparison between pointers of the same type makes sense,
\nthis commit extends reg_not_null(). W\/o such an extension e.g.
\nverifier_jeq_infer_not_null\/null_ptr_to_map_value fails.<\/p>\n
reg_not_null() can be extended further, but I deem that out of scope
\nfor the fix at hand. Explicit base_type(…) != PTR_TO_BTF_ID
\nchecks in the check_cond_jmp_op() can be removed with migration to
\nreg_not_null(), but that is a behavioural change, as the special case
\nwould start matching for PTR_TO_BTF_ID that is also is_trusted_reg().
\nI omit the behavioural change from this commit.<\/p>\n","protected":false},"excerpt":{"rendered":"
In the Linux kernel, the following vulnerability has been resolved:<\/p>\n
bpf: check_cond_jmp_op(): properly infer if register is null<\/p>\n
Nicholas Carlini reported a bug when verifier can incorrectly infer
\nt<\/p>\n","protected":false},"template":"","meta":[],"categories":[],"tags":[],"class_list":["post-29288","vuln","type-vuln","status-publish","hentry"],"_links":{"self":[{"href":"https:\/\/www.54iter.cn\/index.php?rest_route=\/wp\/v2\/vuln\/29288","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=29288"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.54iter.cn\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=29288"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.54iter.cn\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=29288"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}