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Date: Tue, 15 Feb 2022 12:48:12 +0100
From: "Jason A. Donenfeld" <Jason@...c4.com>
To: linux-kernel@...r.kernel.org
Cc: "Jason A. Donenfeld" <Jason@...c4.com>,
Theodore Ts'o <tytso@....edu>,
Dominik Brodowski <linux@...inikbrodowski.net>
Subject: [PATCH v3] random: deobfuscate irq u32/u64 contributions
In the irq handler, we fill out 16 bytes differently on 32-bit and
64-bit platforms, and for 32-bit vs 64-bit cycle counters, which doesn't
always correspond with the bitness of the platform. Whether or not you
like this strangeness, it is a matter of fact. But it might not be a
fact you well realized until now, because the code that loaded the irq
info into 4 32-bit words was quite confusing. Instead, this commit
makes everything explicit by having separate (compile-time) branches for
32-bit and 64-bit types.
Cc: Theodore Ts'o <tytso@....edu>
Reviewed-by: Dominik Brodowski <linux@...inikbrodowski.net>
Signed-off-by: Jason A. Donenfeld <Jason@...c4.com>
---
Changes v2->v3:
- cycles_t is sometimes an `unsigned long long`, even on 32-bit x86, so
separate that contribution from the other one.
- rebased on the latest.
drivers/char/random.c | 49 ++++++++++++++++++++++++-------------------
1 file changed, 28 insertions(+), 21 deletions(-)
diff --git a/drivers/char/random.c b/drivers/char/random.c
index 9714d9f05a84..6a2c7db94417 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -283,7 +283,10 @@ static void mix_pool_bytes(const void *in, size_t nbytes)
}
struct fast_pool {
- u32 pool[4];
+ union {
+ u32 pool32[4];
+ u64 pool64[2];
+ };
unsigned long last;
u16 reg_idx;
u8 count;
@@ -294,10 +297,10 @@ struct fast_pool {
* collector. It's hardcoded for an 128 bit pool and assumes that any
* locks that might be needed are taken by the caller.
*/
-static void fast_mix(struct fast_pool *f)
+static void fast_mix(u32 pool[4])
{
- u32 a = f->pool[0], b = f->pool[1];
- u32 c = f->pool[2], d = f->pool[3];
+ u32 a = pool[0], b = pool[1];
+ u32 c = pool[2], d = pool[3];
a += b; c += d;
b = rol32(b, 6); d = rol32(d, 27);
@@ -315,9 +318,8 @@ static void fast_mix(struct fast_pool *f)
b = rol32(b, 16); d = rol32(d, 14);
d ^= a; b ^= c;
- f->pool[0] = a; f->pool[1] = b;
- f->pool[2] = c; f->pool[3] = d;
- f->count++;
+ pool[0] = a; pool[1] = b;
+ pool[2] = c; pool[3] = d;
}
static void process_random_ready_list(void)
@@ -778,29 +780,34 @@ void add_interrupt_randomness(int irq)
struct pt_regs *regs = get_irq_regs();
unsigned long now = jiffies;
cycles_t cycles = random_get_entropy();
- u32 c_high, j_high;
- u64 ip;
if (cycles == 0)
cycles = get_reg(fast_pool, regs);
- c_high = (sizeof(cycles) > 4) ? cycles >> 32 : 0;
- j_high = (sizeof(now) > 4) ? now >> 32 : 0;
- fast_pool->pool[0] ^= cycles ^ j_high ^ irq;
- fast_pool->pool[1] ^= now ^ c_high;
- ip = regs ? instruction_pointer(regs) : _RET_IP_;
- fast_pool->pool[2] ^= ip;
- fast_pool->pool[3] ^=
- (sizeof(ip) > 4) ? ip >> 32 : get_reg(fast_pool, regs);
- fast_mix(fast_pool);
+ if (sizeof(cycles) == 8)
+ fast_pool->pool64[0] ^= cycles ^ rol64(now, 32) ^ irq;
+ else {
+ fast_pool->pool32[0] ^= cycles ^ irq;
+ fast_pool->pool32[1] ^= now;
+ }
+
+ if (sizeof(unsigned long) == 8)
+ fast_pool->pool64[1] ^= regs ? instruction_pointer(regs) : _RET_IP_;
+ else {
+ fast_pool->pool32[2] ^= regs ? instruction_pointer(regs) : _RET_IP_;
+ fast_pool->pool32[3] ^= get_reg(fast_pool, regs);
+ }
+
+ fast_mix(fast_pool->pool32);
+ ++fast_pool->count;
if (unlikely(crng_init == 0)) {
if (fast_pool->count >= 64 &&
- crng_fast_load(fast_pool->pool, sizeof(fast_pool->pool)) > 0) {
+ crng_fast_load(fast_pool->pool32, sizeof(fast_pool->pool32)) > 0) {
fast_pool->count = 0;
fast_pool->last = now;
if (spin_trylock(&input_pool.lock)) {
- _mix_pool_bytes(&fast_pool->pool, sizeof(fast_pool->pool));
+ _mix_pool_bytes(&fast_pool->pool32, sizeof(fast_pool->pool32));
spin_unlock(&input_pool.lock);
}
}
@@ -814,7 +821,7 @@ void add_interrupt_randomness(int irq)
return;
fast_pool->last = now;
- _mix_pool_bytes(&fast_pool->pool, sizeof(fast_pool->pool));
+ _mix_pool_bytes(&fast_pool->pool32, sizeof(fast_pool->pool32));
spin_unlock(&input_pool.lock);
fast_pool->count = 0;
--
2.35.0
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