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charset=UTF-8 Content-Transfer-Encoding: 7bit On 13/09/2024 1:45 pm, Jan Beulich wrote: > On 13.09.2024 13:09, Andrew Cooper wrote: >> From: Demi Marie Obenour >> >> The AMD container format has no formal spec. It is, at best, precision >> guesswork based on AMD's prior contributions to open source projects. The >> Equivalence Table has both an explicit length, and an expectation of having a >> NULL entry at the end. >> >> Xen was sanity checking the NULL entry, but without confirming that an entry >> was present, resulting in a read off the front of the buffer. With some >> manual debugging/annotations this manifests as: >> >> (XEN) *** Buf ffff83204c00b19c, eq ffff83204c00b194 >> (XEN) *** eq: 0c 00 00 00 44 4d 41 00 00 00 00 00 00 00 00 00 aa aa aa aa >> ^-Actual buffer-------------------^ >> (XEN) *** installed_cpu: 000c >> (XEN) microcode: Bad equivalent cpu table >> (XEN) Parsing microcode blob error -22 >> >> When loaded by hypercall, the 4 bytes interpreted as installed_cpu happen to >> be the containing struct ucode_buf's len field, and luckily will be nonzero. >> >> When loaded at boot, it's possible for the access to #PF if the module happens >> to have been placed on a 2M boundary by the bootloader. Under Linux, it will >> commonly be the end of the CPIO header. >> >> Drop the probe of the NULL entry; Nothing else cares. A container without one >> is well formed, insofar that we can still parse it correctly. With this >> dropped, the same container results in: >> >> (XEN) microcode: couldn't find any matching ucode in the provided blob! >> >> Fixes: 4de936a38aa9 ("x86/ucode/amd: Rework parsing logic in cpu_request_microcode()") >> Signed-off-by: Demi Marie Obenour >> Signed-off-by: Andrew Cooper > Reviewed-by: Jan Beulich Thanks. > I wonder though about scan_equiv_cpu_table(): Should it perhaps complain > if it doesn't find a null entry? And when it find ones, but that's not > last? I'm erring on the "be liberal on what you accept" side of things. scan_equiv_cpu_table() is a relic of Fam10h systems (the last time that two different CPU steppings shared a ucode blob), and recent changes to Linux suggest that AMD have retired the concept. The only thing we do in scan_equiv_cpu_table() is see if there's a mapping for the current system, and we loop over all entries, even if that happens to be 0. ~Andrew