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Date: Mon, 18 Jan 2021 22:22:56 +0100 From: Bodo Stroesser <bostroesser@...il.com> To: Jason Gunthorpe <jgg@...pe.ca>, Douglas Gilbert <dgilbert@...erlog.com> Cc: linux-scsi@...r.kernel.org, linux-block@...r.kernel.org, target-devel@...r.kernel.org, linux-rdma@...r.kernel.org, linux-kernel@...r.kernel.org, martin.petersen@...cle.com, jejb@...ux.vnet.ibm.com, ddiss@...e.de, bvanassche@....org Subject: Re: [PATCH v6 1/4] sgl_alloc_order: remove 4 GiB limit, sgl_free() warning On 18.01.21 21:24, Jason Gunthorpe wrote: > On Mon, Jan 18, 2021 at 03:08:51PM -0500, Douglas Gilbert wrote: >> On 2021-01-18 1:28 p.m., Jason Gunthorpe wrote: >>> On Mon, Jan 18, 2021 at 11:30:03AM -0500, Douglas Gilbert wrote: >>> >>>> After several flawed attempts to detect overflow, take the fastest >>>> route by stating as a pre-condition that the 'order' function argument >>>> cannot exceed 16 (2^16 * 4k = 256 MiB). >>> >>> That doesn't help, the point of the overflow check is similar to >>> overflow checks in kcalloc: to prevent the routine from allocating >>> less memory than the caller might assume. >>> >>> For instance ipr_store_update_fw() uses request_firmware() (which is >>> controlled by userspace) to drive the length argument to >>> sgl_alloc_order(). If userpace gives too large a value this will >>> corrupt kernel memory. >>> >>> So this math: >>> >>> nent = round_up(length, PAGE_SIZE << order) >> (PAGE_SHIFT + order); >> >> But that check itself overflows if order is too large (e.g. 65). > > I don't reall care about order. It is always controlled by the kernel > and it is fine to just require it be low enough to not > overflow. length is the data under userspace control so math on it > must be checked for overflow. > >> Also note there is another pre-condition statement in that function's >> definition, namely that length cannot be 0. > > I don't see callers checking for that either, if it is true length 0 > can't be allowed it should be blocked in the function > > Jason > A already said, I also think there should be a check for length or rather nent overflow. I like the easy to understand check in your proposed code: if (length >> (PAGE_SHIFT + order) >= UINT_MAX) return NULL; But I don't understand, why you open-coded the nent calculation: nent = length >> (PAGE_SHIFT + order); if (length & ((1ULL << (PAGE_SHIFT + order)) - 1)) nent++; Wouldn't it be better to keep the original line instead: nent = round_up(length, PAGE_SIZE << order) >> (PAGE_SHIFT + order); Or maybe even better: nent = DIV_ROUND_UP(length, PAGE_SIZE << order); I think, combining the above lines results in short and easily readable code: u32 elem_len; if (length >> (PAGE_SHIFT + order) >= UINT_MAX) return NULL; nent = DIV_ROUND_UP(length, PAGE_SIZE << order); if (chainable) { if (check_add_overflow(nent, 1, &nalloc)) return NULL; } else nalloc = nent; Thank you, Bodo
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