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[198.145.64.163]) by smtp.gmail.com with ESMTPSA id z12-20020a1709027e8c00b0016dbaf3ff2esm2038893pla.22.2022.10.07.15.47.44 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 07 Oct 2022 15:47:45 -0700 (PDT) Date: Fri, 7 Oct 2022 15:47:44 -0700 From: Kees Cook To: "Jason A. Donenfeld" Cc: linux-kernel@vger.kernel.org, patches@lists.linux.dev, Andreas Noever , Andrew Morton , Andy Shevchenko , Borislav Petkov , Catalin Marinas , Christoph =?iso-8859-1?Q?B=F6hmwalder?= , Christoph Hellwig , Christophe Leroy , Daniel Borkmann , Dave Airlie , Dave Hansen , "David S . Miller" , Eric Dumazet , Florian Westphal , Greg Kroah-Hartman , "H . Peter Anvin" , Heiko Carstens , Helge Deller , Herbert Xu , Huacai Chen , Hugh Dickins , Jakub Kicinski , "James E . J . Bottomley" , Jan Kara , Jason Gunthorpe , Jens Axboe , Johannes Berg , Jonathan Corbet , Jozsef Kadlecsik , KP Singh , Marco Elver , Mauro Carvalho Chehab , Michael Ellerman , Pablo Neira Ayuso , Paolo Abeni , Peter Zijlstra , Richard Weinberger , Russell King , Theodore Ts'o , Thomas Bogendoerfer , Thomas Gleixner , Thomas Graf , Ulf Hansson , Vignesh Raghavendra , WANG Xuerui , Will Deacon , Yury Norov , dri-devel@lists.freedesktop.org, kasan-dev@googlegroups.com, kernel-janitors@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-block@vger.kernel.org, linux-crypto@vger.kernel.org, linux-doc@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-media@vger.kernel.org, linux-mips@vger.kernel.org, linux-mm@kvack.org, linux-mmc@vger.kernel.org, linux-mtd@lists.infradead.org, linux-nvme@lists.infradead.org, linux-parisc@vger.kernel.org, linux-rdma@vger.kernel.org, linux-s390@vger.kernel.org, linux-um@lists.infradead.org, linux-usb@vger.kernel.org, linux-wireless@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, loongarch@lists.linux.dev, netdev@vger.kernel.org, sparclinux@vger.kernel.org, x86@kernel.org, Jan Kara Subject: Re: [PATCH v4 2/6] treewide: use prandom_u32_max() when possible Message-ID: <202210071241.445289C5@keescook> References: <20221007180107.216067-1-Jason@zx2c4.com> <20221007180107.216067-3-Jason@zx2c4.com> Precedence: bulk X-Mailing-List: patches@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20221007180107.216067-3-Jason@zx2c4.com> On Fri, Oct 07, 2022 at 12:01:03PM -0600, Jason A. Donenfeld wrote: > Rather than incurring a division or requesting too many random bytes for > the given range, use the prandom_u32_max() function, which only takes > the minimum required bytes from the RNG and avoids divisions. I actually meant splitting the by-hand stuff by subsystem, but nearly all of these can be done mechanically too, so it shouldn't be bad. Notes below... > > [...] > diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c > index 92bcc1768f0b..87203429f802 100644 > --- a/arch/arm64/kernel/process.c > +++ b/arch/arm64/kernel/process.c > @@ -595,7 +595,7 @@ unsigned long __get_wchan(struct task_struct *p) > unsigned long arch_align_stack(unsigned long sp) > { > if (!(current->personality & ADDR_NO_RANDOMIZE) && randomize_va_space) > - sp -= get_random_int() & ~PAGE_MASK; > + sp -= prandom_u32_max(PAGE_SIZE); > return sp & ~0xf; > } > @mask@ expression MASK; @@ - (get_random_int() & ~(MASK)) + prandom_u32_max(MASK) > diff --git a/arch/loongarch/kernel/vdso.c b/arch/loongarch/kernel/vdso.c > index f32c38abd791..8c9826062652 100644 > --- a/arch/loongarch/kernel/vdso.c > +++ b/arch/loongarch/kernel/vdso.c > @@ -78,7 +78,7 @@ static unsigned long vdso_base(void) > unsigned long base = STACK_TOP; > > if (current->flags & PF_RANDOMIZE) { > - base += get_random_int() & (VDSO_RANDOMIZE_SIZE - 1); > + base += prandom_u32_max(VDSO_RANDOMIZE_SIZE); > base = PAGE_ALIGN(base); > } > @minus_one@ expression FULL; @@ - (get_random_int() & ((FULL) - 1) + prandom_u32_max(FULL) > diff --git a/arch/parisc/kernel/vdso.c b/arch/parisc/kernel/vdso.c > index 63dc44c4c246..47e5960a2f96 100644 > --- a/arch/parisc/kernel/vdso.c > +++ b/arch/parisc/kernel/vdso.c > @@ -75,7 +75,7 @@ int arch_setup_additional_pages(struct linux_binprm *bprm, > > map_base = mm->mmap_base; > if (current->flags & PF_RANDOMIZE) > - map_base -= (get_random_int() & 0x1f) * PAGE_SIZE; > + map_base -= prandom_u32_max(0x20) * PAGE_SIZE; > > vdso_text_start = get_unmapped_area(NULL, map_base, vdso_text_len, 0, 0); > These are more fun, but Coccinelle can still do them with a little Pythonic help: // Find a potential literal @literal_mask@ expression LITERAL; identifier randfunc =~ "get_random_int|prandom_u32|get_random_u32"; position p; @@ (randfunc()@p & (LITERAL)) // Add one to the literal. @script:python add_one@ literal << literal_mask.LITERAL; RESULT; @@ if literal.startswith('0x'): value = int(literal, 16) + 1 coccinelle.RESULT = cocci.make_expr("0x%x" % (value)) elif literal[0] in '123456789': value = int(literal, 10) + 1 coccinelle.RESULT = cocci.make_expr("%d" % (value)) else: print("I don't know how to handle: %s" % (literal)) // Replace the literal mask with the calculated result. @plus_one@ expression literal_mask.LITERAL; position literal_mask.p; expression add_one.RESULT; identifier FUNC; @@ - (FUNC()@p & (LITERAL)) + prandom_u32_max(RESULT) > diff --git a/drivers/mtd/tests/stresstest.c b/drivers/mtd/tests/stresstest.c > index cb29c8c1b370..d2faaca7f19d 100644 > --- a/drivers/mtd/tests/stresstest.c > +++ b/drivers/mtd/tests/stresstest.c > @@ -45,9 +45,8 @@ static int rand_eb(void) > unsigned int eb; > > again: > - eb = prandom_u32(); > /* Read or write up 2 eraseblocks at a time - hence 'ebcnt - 1' */ > - eb %= (ebcnt - 1); > + eb = prandom_u32_max(ebcnt - 1); > if (bbt[eb]) > goto again; > return eb; This can also be done mechanically: @multi_line@ identifier randfunc =~ "get_random_int|prandom_u32|get_random_u32"; identifier RAND; expression E; @@ - RAND = randfunc(); ... when != RAND - RAND %= (E); + RAND = prandom_u32_max(E); @collapse_ret@ type TYPE; identifier VAR; expression E; @@ { - TYPE VAR; - VAR = (E); - return VAR; + return E; } @drop_var@ type TYPE; identifier VAR; @@ { - TYPE VAR; ... when != VAR } > diff --git a/fs/ext2/ialloc.c b/fs/ext2/ialloc.c > index 998dd2ac8008..f4944c4dee60 100644 > --- a/fs/ext2/ialloc.c > +++ b/fs/ext2/ialloc.c > @@ -277,8 +277,7 @@ static int find_group_orlov(struct super_block *sb, struct inode *parent) > int best_ndir = inodes_per_group; > int best_group = -1; > > - group = prandom_u32(); > - parent_group = (unsigned)group % ngroups; > + parent_group = prandom_u32_max(ngroups); > for (i = 0; i < ngroups; i++) { > group = (parent_group + i) % ngroups; > desc = ext2_get_group_desc (sb, group, NULL); Okay, that one is too much for me -- checking that group is never used after the assignment removal is likely possible, but beyond my cocci know-how. :) > diff --git a/fs/ext4/ialloc.c b/fs/ext4/ialloc.c > index f73e5eb43eae..36d5bc595cc2 100644 > --- a/fs/ext4/ialloc.c > +++ b/fs/ext4/ialloc.c > @@ -463,10 +463,9 @@ static int find_group_orlov(struct super_block *sb, struct inode *parent, > hinfo.hash_version = DX_HASH_HALF_MD4; > hinfo.seed = sbi->s_hash_seed; > ext4fs_dirhash(parent, qstr->name, qstr->len, &hinfo); > - grp = hinfo.hash; > + parent_group = hinfo.hash % ngroups; > } else > - grp = prandom_u32(); > - parent_group = (unsigned)grp % ngroups; > + parent_group = prandom_u32_max(ngroups); > for (i = 0; i < ngroups; i++) { > g = (parent_group + i) % ngroups; > get_orlov_stats(sb, g, flex_size, &stats); Much less easy mechanically. :) > diff --git a/lib/test_hexdump.c b/lib/test_hexdump.c > index 0927f44cd478..41a0321f641a 100644 > --- a/lib/test_hexdump.c > +++ b/lib/test_hexdump.c > @@ -208,7 +208,7 @@ static void __init test_hexdump_overflow(size_t buflen, size_t len, > static void __init test_hexdump_overflow_set(size_t buflen, bool ascii) > { > unsigned int i = 0; > - int rs = (prandom_u32_max(2) + 1) * 16; > + int rs = prandom_u32_max(2) + 1 * 16; > > do { > int gs = 1 << i; This looks wrong. Cocci says: - int rs = (get_random_int() % 2 + 1) * 16; + int rs = (prandom_u32_max(2) + 1) * 16; > diff --git a/lib/test_vmalloc.c b/lib/test_vmalloc.c > index 4f2f2d1bac56..56ffaa8dd3f6 100644 > --- a/lib/test_vmalloc.c > +++ b/lib/test_vmalloc.c > @@ -151,9 +151,7 @@ static int random_size_alloc_test(void) > int i; > > for (i = 0; i < test_loop_count; i++) { > - n = prandom_u32(); > - n = (n % 100) + 1; > - > + n = prandom_u32_max(n % 100) + 1; > p = vmalloc(n * PAGE_SIZE); > > if (!p) This looks wrong. Cocci says: - n = prandom_u32(); - n = (n % 100) + 1; + n = prandom_u32_max(100) + 1; > @@ -293,16 +291,12 @@ pcpu_alloc_test(void) > return -1; > > for (i = 0; i < 35000; i++) { > - unsigned int r; > - > - r = prandom_u32(); > - size = (r % (PAGE_SIZE / 4)) + 1; > + size = prandom_u32_max(PAGE_SIZE / 4) + 1; > > /* > * Maximum PAGE_SIZE > */ > - r = prandom_u32(); > - align = 1 << ((r % 11) + 1); > + align = 1 << (prandom_u32_max(11) + 1); > > pcpu[i] = __alloc_percpu(size, align); > if (!pcpu[i]) > @@ -393,14 +387,11 @@ static struct test_driver { > > static void shuffle_array(int *arr, int n) > { > - unsigned int rnd; > int i, j; > > for (i = n - 1; i > 0; i--) { > - rnd = prandom_u32(); > - > /* Cut the range. */ > - j = rnd % i; > + j = prandom_u32_max(i); > > /* Swap indexes. */ > swap(arr[i], arr[j]); Yup, agrees with Cocci on these. -- Kees Cook