> BPR, or Boot Process Register, was a feature implemented in iOS 14 in order to additionally secure devices from bootROM based attacks. Crucially, it restricts data access when a device is booted directly from DFU mode, which is required by both checkm8 and usbliter8. In iOS 14 and 15, this manifested as the requirement to disable your passcode when jailbreaking A11 devices with checkra1n/palera1n, and is the reason why A11 devices must be first erased if they previously had a passcode before jailbreaking with palera1n. A10 devices were not affected by this as they had a SEP exploit, known as blackbird, which prevented this issue from arising. We do not have a SEP exploit for A11 and newer.
Right now we only have a reliable jailbreak (checkm8) for up to iOS 18 (and that's only thanks to one iPad model). Some app developers are pretty aggressive about dropping support for older iOS versions.
This affects iPhone XR, XS, 11, SE 2nd gen, and a smattering of iPads. Many of these devices got the iOS 27 beta and will likely see future iOS versions for at least another year or two.
Edit: here's the affected iPads:
* iPad Pro 11" (gen 1-2)
* iPad Pro 12.9" (gen 3-4)
* iPad mini (gen 5)
* iPad Air (gen 3)
* iPad (gen 8-9)
(well, to be honest this is a bad example to show the system is closed, because copy and paste was difficult on linux too during wayland)
Some were dumped via known exploits, but I don't know how A12/A13 were dumped in the first place. I'd guess someone got code exec via fault injection and dumped it out that way, or perhaps just a privately known vuln.
iBoot source code has also been leaked, in the past.
If not, you need to have unbrick-capable DFU straight in BootROM.
Which typically means: ROM code that carries an entire USB stack, as well as means of validating and booting executables from the USB stack.
An alternative would be to have BootROM recovery off MicroSD, but, iPhone lmao. They didn't chase the trend of "no expandable storage" - they created it.
> Upon receiving a fourth Setup transaction, the DMA base address gets reset to its starting position before writing, akin to a ring buffer mechanism.
> After writing each received packet, the controller increments DOEPDMA by the size of data written. The reset operation is implemented by decrementing DOEPDMA by 24.
> The core issue arises because the controller also accepts smaller packets (though always stores in 4-byte chunks).
> Since the pointer increment does not match the fixed decrement amount, we end up with a buffer underflow primitive in 12-byte steps.
so the problem is directly in the hardware, not in driver
what kind of defense would work against such bugs?
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wait, am I understanding it right that DMA access was given directly to the stack??
However, Boot ROM on these two chips does not program it; Apple probably felt that it was an unnecessary technical risk to do so. The Boot ROM code was well-verified and unlikely to contain bugs like buffer overflows. But nobody expected a hardware bug :)
The USB controller has access to it, but it only increments it and decrements.
By sending multiple packets that are smaller than typical, we can trick the USB controller to decrement the base pointer by more than it should, getting to underflow.
It so happens that on A12, the DMA buffer is after the USB task stack, so getting it to decrement by enough will get it to point to the task stack, where we can then write to LR and control where some function on the stack will eventually return to.
Say a direct copy of the ball nearest the cliff is sent to the back of the line of balls and takes the place of the first ball. The formerly first ball becomes the second, the second becomes the third, and the fourth falls off the cliff.
DOEPDMA works the same way.
https://support.apple.com/guide/iphone/force-restart-iphone-...