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Mon, 29 Dec 2025 07:47:25 -0800 (PST) MIME-Version: 1.0 References: <20251220151019.19473-1-klourencodev@gmail.com> In-Reply-To: From: Kevin Lourenco Date: Mon, 29 Dec 2025 16:47:14 +0100 X-Gm-Features: AQt7F2qHNph7awGIazVvhT9O8t9lqmAwXQ8rcxtnj37strCnccnAzIFxse9g_wI Message-ID: Subject: Re: [PATCH] mm/memtest: prevent arithmetic underflow in end pointer calculation To: Mike Rapoport Cc: linux-mm@kvack.org Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Rspamd-Server: rspam12 X-Rspamd-Queue-Id: 742ED10000F X-Stat-Signature: d1mx31g5gmyh6xyfbn81pdc3stpyf45y X-Rspam-User: X-HE-Tag: 1767023247-975293 X-HE-Meta: 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 8ZXSJsai Nguo+BopLXxsmufspXssyyhq73SAvsfKjQZXPTfIo/BJFbUOIET2+UjdR+BopkqCTzApHdmSNULvOHtg6woN4Y/MnEY5X0n7Yr0ud X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: Hi Mike, > Is it exactly ~0? Not exactly - it's UINT64_MAX - (offset - size), but since offset is at most 7 bytes (from ALIGN), the result is in the range [~0 - 7, ~0], which after division by 8 still yields ~2^61 iterations. Either way, if it happens, it's not great, WARN_ON_ONCE usage sounds good, I'll send a v2 patch. Thanks for the feedback! Le dim. 28 d=C3=A9c. 2025 =C3=A0 20:56, Mike Rapoport a = =C3=A9crit : > > On Sat, Dec 20, 2025 at 04:10:19PM +0100, klourencodev@gmail.com wrote: > > From: Kevin Lourenco > > > > The computation of the loop end pointer can underflow when size is > > smaller than the alignment offset: > > > > (size - (start_phys_aligned - start_phys)) > > > > If size < offset, the unsigned subtraction wraps to ~0, causing a > > Is it exactly ~0? > > > massive loop iteration that writes far beyond the intended region, > > leading to memory corruption during early boot. > > > > While unlikely in practice (memblock regions are typically KB/MB), cost= is negligible > > (one comparison), but it prevents catastrophic memory corruption in > > edge cases. > > > > Signed-off-by: Kevin Lourenco > > --- > > mm/memtest.c | 9 +++++++-- > > 1 file changed, 7 insertions(+), 2 deletions(-) > > > > diff --git a/mm/memtest.c b/mm/memtest.c > > index c2c609c39119..d86c41f1c189 100644 > > --- a/mm/memtest.c > > +++ b/mm/memtest.c > > @@ -41,12 +41,17 @@ static void __init memtest(u64 pattern, phys_addr_t= start_phys, phys_addr_t size > > { > > u64 *p, *start, *end; > > phys_addr_t start_bad, last_bad; > > - phys_addr_t start_phys_aligned; > > + phys_addr_t start_phys_aligned, offset; > > const size_t incr =3D sizeof(pattern); > > > > start_phys_aligned =3D ALIGN(start_phys, incr); > > start =3D __va(start_phys_aligned); > > - end =3D start + (size - (start_phys_aligned - start_phys)) / incr= ; > > I believe VM_WARN_ON_ONCE(size < start_phys_aligned - start_phys) is > sufficient here to detect those theoretical edge cases. > > > + > > + offset =3D start_phys_aligned - start_phys; > > + if (size < offset) > > + return; > > + > > + end =3D start + (size - offset) / incr; > > start_bad =3D 0; > > last_bad =3D 0; > > > > -- > > 2.47.3 > > > > -- > Sincerely yours, > Mike.