Subject: Re: powerpc/nohash: fix undefined behaviour when testing page size support

From: Michael Ellerman <[email-masked]>
Date: Thu, 11 Oct 2018 19:35:41 +1100 (AEDT)

On Mon, 2018-10-01 at 06:21:51 UTC, Daniel Axtens wrote:
> When enumerating page size definitions to check hardware support,
> we construct a constant which is (1U << (def->shift - 10)).
> 
> However, the array of page size definitions is only initalised for
> various MMU_PAGE_* constants, so it contains a number of 0-initialised
> elements with def->shift == 0. This means we end up shifting by a
> very large number, which gives the following UBSan splat:
> 
> ================================================================================
> UBSAN: Undefined behaviour in /home/dja/dev/linux/linux/arch/powerpc/mm/tlb_nohash.c:506:21
> shift exponent 4294967286 is too large for 32-bit type 'unsigned int'
> CPU: 0 PID: 0 Comm: swapper Not tainted 4.19.0-rc3-00045-ga604f927b012-dirty #6
> Call Trace:
> [c00000000101bc20] [c000000000a13d54] .dump_stack+0xa8/0xec (unreliable)
> [c00000000101bcb0] [c0000000004f20a8] .ubsan_epilogue+0x18/0x64
> [c00000000101bd30] [c0000000004f2b10] .__ubsan_handle_shift_out_of_bounds+0x110/0x1a4
> [c00000000101be20] [c000000000d21760] .early_init_mmu+0x1b4/0x5a0
> [c00000000101bf10] [c000000000d1ba28] .early_setup+0x100/0x130
> [c00000000101bf90] [c000000000000528] start_here_multiplatform+0x68/0x80
> ================================================================================
> 
> Fix this by first checking if the element exists (shift != 0) before
> constructing the constant.
> 
> Signed-off-by: Daniel Axtens <[email-masked]>

Applied to powerpc next, thanks.

https://git.kernel.org/powerpc/c/f5e284803a7206d43e26f9ffcae5de

cheers