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Wed, 12 Feb 2025 17:28:19 -0800 (PST) MIME-Version: 1.0 References: <20241022015913.3524425-1-samuel.holland@sifive.com> <20241022015913.3524425-2-samuel.holland@sifive.com> In-Reply-To: From: Andrey Konovalov Date: Thu, 13 Feb 2025 02:28:08 +0100 X-Gm-Features: AWEUYZn2XkAKpo5fHBIg4DaefcDCWc7cwclFMrZ0d3cOTpLW86BXbO6cjB81ilc Message-ID: Subject: Re: [PATCH v2 1/9] kasan: sw_tags: Use arithmetic shift for shadow computation To: Maciej Wieczor-Retman Cc: Samuel Holland , Palmer Dabbelt , linux-riscv@lists.infradead.org, Andrey Ryabinin , Alexander Potapenko , Dmitry Vyukov , Vincenzo Frascino , kasan-dev@googlegroups.com, llvm@lists.linux.dev, Catalin Marinas , linux-kernel@vger.kernel.org, linux-mm@kvack.org, Alexandre Ghiti , Will Deacon , Evgenii Stepanov , Andrew Morton , linux-arm-kernel@lists.infradead.org Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Rspam-User: X-Rspamd-Queue-Id: 36F5E1A0006 X-Stat-Signature: phcgk3fnoua34ztykzed6mj6kb7ofcdp X-Rspamd-Server: rspam03 X-HE-Tag: 1739410101-575558 X-HE-Meta: U2FsdGVkX1+TAflv65z5fuBKtpngn1mcmDFgR4+5VhVBWwYUdbfZ/ZjhbfXEULESGtzRmF12djA9iIEE0pJM+g5oQRGWySCeKyTq2lEOduKZk16M7SB6W4KzEjKkl2MK5jitCdROcPQ080iupt6tgdfUXIdW4TANMd1cwoVdvkDoCTm88tMPQR2BHzNTZ1TG+QeR8Svt+nv3XjkPfZMjJve8jHVQyr/IPY7WzWbjwZ6H7/kALaSXdFj1rQKZe4TWrHTVSG7tJNVRjBuGu3utaT4OPzDI4Lw1kDYDhha7DglN6RasBiwe85pYBb+5N3EXi5ZmrflNDR/TaGNsH/ms+1aeurw4kzx9KQw0f9+VnBfylxWEH4pGiTBVVIm/9xiaU9PbH0NDIIHjdqG17hD2XazpZOSd/agk1/H2Q/3WIKDbME+OmRtiiNBGEOB1gQq+Z/DXbS/rhAAWFwp+F8o4j6Mvl6xZBhxHegKi4RwAhxvkw1h7Eu2iPhr8NrvJrKMINEPP0FPfHUcrEjYN0n+UCRxvdVRpbTqSaCpXg4yHnue91V8qSHIJNkKEUKLurJT2mkNqsWYk1vmiAp7nICh65zLkfl/4k1dLrxvniKz9oan78W8kXCw1UHXrBaQxcEjKM//rcFQ5Xbk+gPCzPFTNSBtgIeiYU0JMz6IwWZQeUOX6uHfbFyCRrHEQbwN8FthSxi0ASqVEORJb30uQjt8Z55r673ZYd9I+xazDzGmMeFJMq6wQ9DL7dQpo1Y9dvM5v28UdurVci1QO4Sd/tCx0irdmTFIp9fIdKczs8sYm14T0L953mH2pPAF4kRcyMpAScRWK75I8cFQFwKDSG3AdHtVUBsZp9Xh6bdMsJ/hUTWlyhcrOzWg2bUiM+CUOO9czm9/DZ1lEqySX3GZ4bezBR+s8ajEdGrCmYkX9Cl5gedEkZaj89bdK8lP+0CLGjIaZtnIzNITzXgFnSfreGaY B+L6AYnC 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000038, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Thu, Feb 13, 2025 at 2:21=E2=80=AFAM Andrey Konovalov wrote: > > On Tue, Feb 11, 2025 at 7:07=E2=80=AFPM Maciej Wieczor-Retman > wrote: > > > > I did some experiments with multiple addresses passed through > > kasan_mem_to_shadow(). And it seems like we can get almost any address = out when > > we consider any random bogus pointers. > > > > I used the KASAN_SHADOW_OFFSET from your example above. Userspace addre= sses seem > > to map to the range [KASAN_SHADOW_OFFSET - 0xffff8fffffffffff]. Then go= ing > > through non-canonical addresses until 0x0007ffffffffffff we reach the e= nd of > > kernel LA and we loop around. Then the addresses seem to go from 0 unti= l we > > again start reaching the kernel space and then it maps into the proper = shadow > > memory. > > > > It gave me the same results when using the previous version of > > kasan_mem_to_shadow() so I'm wondering whether I'm doing this experimen= t > > incorrectly or if there aren't any addresses we can rule out here? > > By the definition of the shadow mapping, if we apply that mapping to > the whole 64-bit address space, the result will only contain 1/8th > (1/16th for SW/HW_TAGS) of that space. > > For example, with the current upstream value of KASAN_SHADOW_OFFSET on > x86 and arm64, the value of the top 3 bits (4 for SW/HW_TAGS) of any > shadow address are always the same: KASAN_SHADOW_OFFSET's value is > such that the shadow address calculation never overflows. Addresses > that have a different value for those top 3 bits are the once we can > rule out. Eh, scratch that, the 3rd bit from the top changes, as KASAN_SHADOW_OFFSET is not a that-well-aligned value, the overall size of the mapping holds. > The KASAN_SHADOW_OFFSET value from my example does rely on the > overflow (arguably, this makes things more confusing [1]). But still, > the possible values of shadow addresses should only cover 1/16th of > the address space. > > So whether the address belongs to that 1/8th (1/16th) of the address > space is what we want to check in kasan_non_canonical_hook(). > > The current upstream version of kasan_non_canonical_hook() actually > does a simplified check by only checking for the lower bound (e.g. for > x86, there's also an upper bound: KASAN_SHADOW_OFFSET + > (0xffffffffffffffff >> 3) =3D=3D 0xfffffbffffffffff), so we could improve > it. > > [1] https://bugzilla.kernel.org/show_bug.cgi?id=3D218043